To answer this question, the user needs to know the basic concepts of functions in programming.

Public and Private functions are both used in classes in Object-Oriented Programming.

Public functions are those that can be accessed by any other code outside of the class in which they are defined. That means that any other code that creates an object of this class can use the public function.

On the other hand, Private functions can only be accessed by the code within the same class. No other code outside of that class can access the private function.

Now, let's go through each option and explain why it is right or wrong:

A. Public functions are free, you have to buy private ones: This option is incorrect because the cost of the function does not depend on whether it is public or private. The access level of a function does not affect its cost.

B. Public functions are the only ones you can download: This option is incorrect because both public and private functions can be downloaded. The downloadability of a function is not related to its access level.

C. Public functions can be used by anyone, private can only be used by other code in the class you are writing: This option is correct. Public functions are accessible by any other code outside of the class in which they are defined, whereas private functions can only be accessed by the code within the same class.

D. Public functions can’t be used: This option is incorrect because public functions can be used by other code outside of the class in which they are defined.

Therefore, the correct answer is:

The Answer is: C. Public functions can be used by anyone, private can only be used by other code in the class you are writing.