Explanation
direct cast. If object is not of the given type, an InvalidCastException is thrown.
Incorrect:
Not B: If obj is not of the given type, result is null.
Not C: If obj is not of a given type, result is false.
Not D: This simply check the variable during runtime. It will not throw an exception.
Reference: http://msdn.microsoft.com/en-us/library/ms173105.aspx