HTML generation in C#

Before we move to the generation part, let's talk about your html...

<p>
    <!-- questionSection -->
    <div class="alert alert-info" >
        <strong style='color:#000000'>question</strong>
    </div>
</p>
<p>
    <!-- answerSection: correct -->
    <p style='margin: 10px 0px 5px 400px;'>The correct answer was:</p>
    <div class='alert alert-success review-correct'>
        <strong>good answer</strong>
    </div>
    
    <br/>
    You had selected: <span style='color:#a94442'>bad answer</span>

    <!-- answerSection: wrong -->
    <p style='margin: 10px 0px 5px 400px;'>Your correct answer:</p>
    <div class='alert alert-success review-correct'><strong>qwe</strong></div>
</p>
<p>
    <!-- reviewSection -->
    <!-- ForQuiz(quiz) -->
</p>

1. <p> can only nest phrasing content. <div> and <p> both belongs to flow content and palpable content. Therefore, you can't nested these 2 under <p> element. For the definition of a phrasing content consult this answer.
2. Personally, I would prefer using <p> to separate block of text, instead of using a <br /> to insert a line-break.

    <!-- Question Section -->
    <div class="alert alert-info" >
        <strong style='color:#000000'>question</strong>
    </div>

    <!-- Answer Section: correct -->
    <p style='margin: 10px 0px 5px 400px;'>The correct answer was:</p>
    <div class='alert alert-success review-correct'>
        <strong>good answer</strong>
    </div>
    
    <p>You had selected: <span style='color:#a94442'>bad answer</span></p>

    <!-- Answer Section: wrong -->
    <p style='margin: 10px 0px 5px 400px;'>Your correct answer:</p>
    <div class='alert alert-success review-correct'>
        <strong>the answer</strong>
    </div>

    <!-- Review Section -->
    <!-- ForQuiz(quiz) -->

As for html generation, I believe, it is better to use a templating engine, such as RazorEngine which allows you to write C# code within the template itself :

review.cshtml :

@{
    var correctAnswer = Model.Answer.GetText();
    var userAnswer = Model.UserAnswer == null ? Model.UserAnswer.GetText() : string.Empty;
}

<!-- Question Section -->
<div class="alert alert-info">
    <strong style='color:#000000'>@Model.Question.GetText()</strong>
</div>

<!-- Answer Section -->
@if (userAnswer == correctAnswer)
{
    <p style='margin: 10px 0px 5px 400px;'>The correct answer was:</p>
    <div class='alert alert-success review-correct'>
        <strong>@correctAnswer</strong>
    </div>
    <p>You had selected: <span style='color:#a94442'>@userAnswer</span></p>
}
else
{
    <p style='margin: 10px 0px 5px 400px;'>Your correct answer:</p>
    <div class='alert alert-success review-correct'>
        <strong>@correctAnswer</strong>
    </div>
}

<!-- Review Section -->
<!-- TODO: implement ForQuiz(quiz) in razor syntax... -->

generation code :

public string GetReview(Quiz quiz)
{
    var path = Desktop.GetFile("review.cshtml"); // TODO: fix this path
    var template = File.ReadAllText(path);

    return Engine.Razor.RunCompile(template, path, typeof(Quiz), quiz);
}

EDIT: I noticed you used the null-propagation operator from c# 6. You can also use the new string interpolation to format the html :

public static string GetReview3(Quiz quiz)
{
    var builder = new StringBuilder();
    builder.Append($@"
        <div class='alert alert-info'>
            <strong style='color:#000000'>{quiz.Question.GetText()}</strong>
        </div>
    ");

    string correctAnswer = quiz.Answer.GetText();
    string userAnswer = quiz.UserAnswer?.GetText() ?? string.Empty;
    if (correctAnswer == userAnswer)
    {
        builder.Append($@"
            <p style='margin: 10px 0px 5px 400px;'>The correct answer was:</p>
            <div class='alert alert-success review-correct'>
                <strong>{correctAnswer}</strong>
            </div>
            <p>You had selected: <span style='color:#a94442'>{userAnswer}</span></p>
        ");
    }
    else
    {
        builder.Append($@"
            <p style='margin: 10px 0px 5px 400px;'>Your correct answer:</p>
            <div class='alert alert-success review-correct'>
                <strong>{correctAnswer}</strong>
            </div>
        ");
    }

    builder.Append(ForQuiz(quiz));


    return builder.ToString();
}