To be called object-oriented, a programming language must allow A. functions that return only a single value B. #include files C. inheritance D. All of the above

The correct answer is D. All of the above.

A programming language is said to be object-oriented if it supports the following features:

• Encapsulation: The data and code that manipulates that data are kept together in a single unit called an object. This makes it easier to understand and maintain the code.
• Inheritance: Objects can inherit the properties and methods of other objects. This makes it easier to reuse code and create new objects quickly.
• Polymorphism: Objects can be used in different ways depending on their type. This makes code more flexible and reusable.

Functions that return only a single value and #include files are not specific to object-oriented programming. They can be used in both object-oriented and non-object-oriented programming languages.

Here is a brief explanation of each of the features of object-oriented programming:

• Encapsulation: Encapsulation is the bundling of data and code together into a single unit called an object. This makes it easier to understand and maintain the code. For example, a car object might have data members for its speed, fuel level, and current gear. It might also have methods for accelerating, braking, and changing gears.
• Inheritance: Inheritance is the ability of an object to inherit the properties and methods of another object. This makes it easier to reuse code and create new objects quickly. For example, a sports car object might inherit from a car object. This means that the sports car object would have all of the properties and methods of a car, plus any additional properties and methods that are specific to sports cars.
• Polymorphism: Polymorphism is the ability of an object to be used in different ways depending on its type. This makes code more flexible and reusable. For example, a function that takes a car object as an argument might be able to accelerate the car, brake the car, or change the car’s gear. The specific action that the function takes will depend on the type of car that is passed to the function.