First, select your right answer, then randomly select an index and assign your right answer to the random radio button.
Fill the other radio buttons with random answers.
Tip: store your radio buttons in an list to facilitate this operation. You begin by filling the list with all your radio buttons, then remove them from the list when they're filled with an answer, this way you don't have to handle "which index did I put the right answer in" or "complicated code that references manually the controls by name"
Edit: As pointed out by Alexei Levenkov in another answer, see this thread for more information on how to generate random numbers properly
Assuming a Random random
declared in your app
List<RadioButton> buttons = new List<RadioButton>();
buttons.Add(answer);
buttons.Add(answer2);
buttons.Add(answer3);
buttons.Add(answer4);
int goodAnswerPos = random.Next(buttons.Count);
buttons[goodAnswerPos].Text = "Good Answer";
buttons.RemoveAt(goodAnswerPos);
foreach (RadioButton button in buttons)
{
button.Text = "Randomly Selected Wrong Answer";
}
Storing the control at buttons[goodAnswerPos]
will allow you to know if the user selected the right one when he submits the answer.