Which one of the following keywords causes a compile time error if used in a static method?

The keyword 'this' returns a reference to the current instance of the class containing it. Static methods (or any static member) do not belong to a particular instance. They exist without creating an instance of the class.

A static class is never instantiated. The static keyword on a class enforces that a type not be created with a constructor. In the static class, we access members directly on the type. This eliminates misuse of the class.

A static class cannot have non-static members. All methods, fields and properties in it must also be static.