C# Enums: Nullable or 'Unknown' Value?

Question

If I have a class with an enum member and I want to be able to represent situations where this member is not defined, which is it better?

a) Declare the

Answer 1

Everything changes if the enum is attributed with [Flags] Attribute.

    [Flags]
    enum FlagsEnum
    {
        None = 0,
        First = 1,
        Second = 2,
        Third = 4,
        FirstAndThird = 5
    }