Split string into multiple string separated

You can split by \|?\s+(?=[A-Z]\.) :

string[] substrings = Regex.Split(value, @"\|?\s+(?=[A-Z]\.)");

If it were me, I would create a simple class that holds a question and the possible answers. This way, you can parse your string into a class which can do other fancy things, like display itself correctly, and could contain a property that indicates the index of the correct answer (so you can compare the user's selection with the correct index).

For example, this class has a static Parse method that takes in your string and returns an instance of the class. It also overrides ToString so that it displays the Question and Answers in a nice way:

public class QuestionAndAnswers
{
    public string Question { get; set; }
    public List<string> Answers { get; set; }
    public int IndexOfCorrectAnswer { get; set; }

    public static QuestionAndAnswers Parse(string qaInput)
    {
        if (qaInput == null) throw new ArgumentNullException(nameof(qaInput));

        var result = new QuestionAndAnswers();

        var parts = Regex.Split(qaInput, @"\|?\s+(?=[A-Z]\.)");

        result.Question = parts[0].TrimEnd('.');
        result.Answers = parts.Skip(1).ToList();

        return result;
    }

    public override string ToString()
    {
        return $"{Question}\n  {string.Join("\n  ", Answers)}";
    }
}

Then it can be use something like this:

private static void Main()
{
    var input = "What is Facebook?.| A. Website B. Internet C. Social Network D. Game";

    var qa = QuestionAndAnswers.Parse(input);

    Console.WriteLine(qa);

    Console.WriteLine("\nDone!\nPress any key to exit...");
    Console.ReadKey();
}

Output

Like @Andrey Nasonov's comment said, this is a good application for regular expressions.

The following will get you your split, though I feel like there must be a better way to represent \s(\w.[\w\s]+ repeating four times.

Regex splitter = new Regex(@"(.*)\|\s(\w\.[\w\s]+)\s(\w\.[\w\s]+)\s(\w\.[\w\s]+)\s(\w\.[\w\s]+)");
string[] results = splitter.Split(input);

stringA = results[0];
stringB = results[1];
stringC = results[2];
stringD = results[3];
stringE = results[4];