Mastering C++: A Comprehensive Quiz Based on 'Thinking in C++

The operator+ should be overloaded as a const member function as it prevents modification of the current object's state. Option A is incorrect as the first parameter of an overloaded operator+ should be a reference to an object of the same class. Option C is incorrect as static member functions cannot be called on objects. Option D is incorrect as it returns a reference to an integer, whereas the operator+ should return the modified object. Therefore, option B is the correct way to overload the '+' operator for a class as a const member function.