1. public String registrationNumber;. String is a reference type variable. All others are value type variables.
2. Struct is a value type variable unlike all others which are reference type variables.
3. No, in C# we cannot have multiple inheritance. A class can only inherit from one base class.
4. In C#, an interface must only have function declarations and not definitions. All other statements are correct.
5. If a member variable is declared as protected, it can only be accessed in the classes that inherit from its base parent class.