How to only accept valid options in ReadLine (C#)

This Driving License Exam project would be my 4th code after starting to learn programming.

Program.cs

namespace DrivingLicenseExam
{
    class Program
    {
        static void Main(string[] args)
        {
            Questions firstQuestion = new Questions("1. You may drive off the paved roadway to pass another vehicle: ", "c", "");

            Console.WriteLine(firstQuestion.question);
            Console.WriteLine("----");
            Console.WriteLine("a) - If the shoulder is wide enough to accommodate your vehicle.");
            Console.WriteLine("b) - If the vehicle ahead of you is turning left.");
            Console.WriteLine("c) - Under no circumstances.");
            Console.Write("Your answer is: ");
            firstQuestion.UserAnswer = Console.ReadLine().ToLower();
            if (firstQuestion.UserAnswer == firstQuestion.correctAnswer)
            {
                totalScore++;
            }

        }
    }
} 

Questions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DrivingLicenseExam
{
    class Questions
    {
        public string question;
        public string correctAnswer;
        private string userAnswer;

        public Questions(string aQuestion, string aCorrectAnswer, string aUserAnswer)
        {
            question = aQuestion;
            correctAnswer = aCorrectAnswer;
            UserAnswer = aUserAnswer;
        }

        public string UserAnswer
        {  
           get { return userAnswer; } 
           set {
                    if (value == "a" || value == "b" || value == "c")
                    {
                        userAnswer = value;
                    }
                    else
                    {
                        Console.WriteLine("Invalid Answer. - Please choose from \"a\", \"b\" or \"c\".");

                    }
           }
        }
    }
}

I am using objects to create the questions with the valid answers and I want the user to only be able to enter "a", "b" or "c", otherwise it should say:

Console.WriteLine("Invalid answer.");
Console.WriteLine("Please choose between \"a\", \"b\" or \"c\".");

This is an example from a previous project but I am not sure how to integrate it in this one:

do
    {
        string measureUnit = Console.ReadLine().ToLower();
        if (measureUnit == "km" || measureUnit == "cm" || measureUnit == "m")
        {
            return measureUnit;
        }

        Console.WriteLine("Incorrect measurement");
        Console.Write("Choose: ");
                
    } while (true);

Thank you!

The easiest way I can think of is to create a method to validate and a method to verify the answer within the questions class.

The while loop in the Main program below keeps calling the "ValidateAnswer" method until a valid input is received.

   class Questions
    {
        public string question;
        public string correctAnswer;
        private string userAnswer;

        public Questions(string aQuestion, string aCorrectAnswer)
        {
            question = aQuestion;
            correctAnswer = aCorrectAnswer;
        }

        public bool ValidateAnswer(string aUserAnswer)
        {
            if (aUserAnswer == "a" || aUserAnswer == "b" || aUserAnswer == "c")
            {
                return true;
            }
            Console.WriteLine("Invalid answer.");
            Console.WriteLine("Please choose between \"a\", \"b\" or \"c\".");
            return false;
        }

        public bool VerifyAnswer(string aUserAnswer)
        {
            return (aUserAnswer == correctAnswer);
        }

        static void Main(string[] args)
        {
            Questions firstQuestion = new Questions("1. You may drive off the paved roadway to pass another vehicle: ", "c");

            Console.WriteLine(firstQuestion.question);
            Console.WriteLine("----");
            Console.WriteLine("a) - If the shoulder is wide enough to accommodate your vehicle.");
            Console.WriteLine("b) - If the vehicle ahead of you is turning left.");
            Console.WriteLine("c) - Under no circumstances.");
            Console.Write("Your answer is: ");

            string answer = Console.ReadLine().ToLower();
            while (!firstQuestion.ValidateAnswer(answer))
            {
                answer = Console.ReadLine().ToLower();
            }

            if (firstQuestion.VerifyAnswer(answer))
            {
                totalScore++;
            }

        }
    }

As an addendum to Tom Hazell's fine answer I would make a more generic ValidateAnswer:

public bool ValidateAnswer(string aUserAnswer, string[] possibleAnswers)
{
    if (possibleAnswers.Contains(aUserAnswer)
    {
        return true;
    }
    Console.WriteLine("Invalid answer.");
    var options = string.Join(", ", possibleAnswers);
    Console.WriteLine($"Please choose between {options}");
    return false;
}

The idea is to pass the valid characters, if a question might have more than three answers.

Then you would call it like:

string answer = Console.ReadLine().ToLower();
while (!firstQuestion.ValidateAnswer(answer, new string {"a","b","c"}))
{
    answer = Console.ReadLine().ToLower();
}

Contains requires using System.Linq;.

If you insist on the nicer formulation of the error message, you can do:

var first = string.Join(", ", possibleAnswers.Take(possibleAnswers.Count() - 1));
var last = possibleAnswers[possibleAnswers.Count() - 1];

Console.WriteLine($"Please choose between {first} and {last}.");

All of this only works if possibleAnswers contains more than one element, but having 1 or 0 answers to a question wouldn't make sense.