Final answer:
Users can change private variables in Object-Oriented Programming (OOP) through public member functions, which allow controlled access to a class's private data while maintaining encapsulation.
Step-by-step explanation:
The question pertains to how users can change private variables in the context of Object-Oriented Programming (OOP). The correct answer is c) Through public member functions. In OOP, private variables are encapsulated within a class and are not directly accessible from outside the class. To modify these private variables, public member functions are provided. These functions are part of the class's public interface and allow controlled access to the class's private data. Option a) Using friend functions, is not a general method because friend functions are specific to C++ and break encapsulation. Option b) is incorrect as it violates the principle of encapsulation by accessing private members directly from other classes, which is not allowed. Likewise, option d) is not applicable as it suggests there is no method to change private variables, which is not true. Therefore, the correct method is through public member functions that are designed to interact with the private data safely and according to the rules defined by the class.