A pure virtual function is a virtual function that

        A virtual function is a member function specified in the base class and re-defined by the derived class.

        It is impossible to instantiate abstract classes. All inherited pure virtual functions must be overridden/implemented by derived classes. If they don't, they'll become abstract as well. A class can declare a pure virtual function with an implementation, which is an interesting 'feature' of C++.

        If the derived class is not abstract, a pure virtual function method is a virtual function that must be implemented by the derived class. Abstract classes are those that include only virtual methods and cannot be instantiated directly.

Hence, option D is the correct answer i.e. all of the above