Before we start, take a look at this comment of yours. It's a prime example of why you shouldn't always trust the comments.

'This gets the text that comes before the : 
'Example Company:TextArea would return TextArea. 
controlOption = textSelection.Substring(0, textSelection.IndexOf(":"c))

The text before : is in fact Company ;)

Now, the name of the method BuildHtmlControlResponsive, IMHO "responsive" is referring the word "build. The builder is run in responsive mode and creates a HtmlControl.

Public Function BuildHtmlControlResponsive(sourceText As String, indexNum As Integer) As String

In reality you're building a ResponsiveHtmlControl. So I suggest you rename the method. I also suggest you rename the parameters. (Please don't abbreviate field/method names)

Public Function BuildResponsiveHtmlControl(command As String, index As Integer) As String

Syntax

You need to define the syntax. This is how I read the rules:

{ labelText~ [ inputType: [ selectOption [;....n] ] ] }

• { } (braces) Required syntax items. Do not type the braces.
• [ ] (brackets) Optional syntax items. Do not type the brackets.
• [;...n] Indicates the preceding item can be repeated n number of times. The occurrences are separated by semicolons.

The name of some of your variables makes no sense. I took the liberty to change all of them. You should also store the character position returned from IndexOf to avoid duplicate calls. And there's a bug in your code. What if there's only one option? Aka. no semicolon?

Test~TextArea: option1

If we apply all the changes/fixes, the code will look like this:

Dim labelText As String = String.Empty
Dim inputType As String = String.Empty
Dim selectOptions As String() = Nothing
Dim position As Integer = command.IndexOf("~")

If (position > -1) Then

    Dim cursor As Integer = (position + 1)

    labelText = command.Substring(0, position)
    position = command.IndexOf(":", cursor)

    If (position > -1) Then

        inputType = command.Substring(cursor, (position - cursor)).ToLower()
        cursor = (position + 1)

        If ((inputType = "select") AndAlso (cursor < command.Length)) Then
            selectOptions = command.Substring(cursor).Split(";"c)
        End If

    End If

End If

HTML

HTML code is usually written in lowercase, so I suggest you change it accordingly.

controlOption.ToUpper()

The input element doesn't have an attribute named Columns. And you cannot have two identical attributes (you have two class attributes).

out.Append("<input type=""text""  class=""form-control"" id=""extField_" & indexNum & """  Columns=""18"" class=""form"" />" & vbCrLf)

Why do you default to an input element if there are only one or less options? It makes no sense. I suggest you return an empty select element.

Case "SELECT"
    If (sourceText.IndexOf(";"c) <> -1) Then
        ...
    Else
        ' text box. Syntax: name
        out.Append("<input type=""text""  class=""form-control"" id=""extField_" & indexNum & """  Columns=""18"" class=""form"" />" & vbCrLf)
    End If

It's pretty clear that you have a maintainability issue, but you also have a readability issue. If speed isn't that big of concern, then I have a great suggestion. VB.Net has a great support for writing XML in the code editor. But please note that XML is not HTML.

Again, if we apply all the changes/fixes, the code will look like this:

Dim elementId As String = String.Format("extField_{0}", index)
Dim labelId As String = String.Format("extLabelField_{0}", index)
Dim element As XElement = Nothing

Select Case inputType
    Case "textarea"
        element = <textarea class="form-control" rows="3" id=<%= elementId %> placeholder=<%= labelText %>></textarea>
    Case "select"
        element = <select id=<%= elementId %>></select>
        Array.ForEach(If(selectOptions, {}), Sub(item) element.Add(<option><%= item %></option>))
    Case Else
        element = <input type="text" id=<%= elementId %> placeholder=<%= labelText %>/>
End Select

Dim root =
    <div class="form-group">
        <label class="control-label col-md-4" for=<%= elementId %> id=<%= labelId %>>Test:</label>
        <div class="col-md-8"></div>
    </div>

root.Element("div").Add(element)

Return root.ToString()

The code looks even better when viewed in Visual Studio.

Unit test

I'm not sure why you use C# for unit testing, but if you can switch to VB.Net, here's how easy it is to write (and read) the expected result (assuming the index = 1234):

Dim command As String = "Test~select:option1;option2;option3;option4"

Dim expectedResult As String =
    <div class="form-group">
        <label class="control-label col-md-4" for="extField_1234" id="extLabelField_1234">Test:</label>
        <div class="col-md-8">
            <select id="extField_1234">
                <option>option1</option>
                <option>option2</option>
                <option>option3</option>
                <option>option4</option>
            </select>
        </div>
    </div>.ToString()

Print screen of Visual Studio code editor:

Source code

Here's the complete method body:

Public Function BuildResponsiveHtmlControl(command As String, index As Integer) As String

    Dim labelText As String = String.Empty
    Dim inputType As String = String.Empty
    Dim selectOptions As String() = {}
    Dim position As Integer = command.IndexOf("~")

    If (position > -1) Then

        Dim cursor As Integer = (position + 1)

        labelText = command.Substring(0, position)
        position = command.IndexOf(":", cursor)

        If (position > -1) Then

            inputType = command.Substring(cursor, (position - cursor)).ToLower()
            cursor = (position + 1)

            If ((inputType = "select") AndAlso (cursor < command.Length)) Then
                selectOptions = command.Substring(cursor).Split(";"c)
            End If

        End If

    End If

    Dim elementId As String = String.Format("extField_{0}", index)
    Dim labelId As String = String.Format("extLabelField_{0}", index)
    Dim element As XElement = Nothing

    Select Case inputType
        Case "textarea"
            element = <textarea class="form-control" rows="3" id=<%= elementId %> placeholder=<%= labelText %>></textarea>
        Case "select"
            element = <select id=<%= elementId %>></select>
            Array.ForEach(If(selectOptions, {}), Sub(item) element.Add(<option><%= item %></option>))
        Case Else
            element = <input type="text" id=<%= elementId %> placeholder=<%= labelText %>/>
    End Select

    Dim root =
        <div class="form-group">
            <label class="control-label col-md-4" for=<%= elementId %> id=<%= labelId %>>Test:</label>
            <div class="col-md-8"></div>
        </div>

    root.Element("div").Add(element)

    Return root.ToString()

End Function