Which of the following is an example of a good global variable?

A good global variable is one that is relevant and necessary for multiple parts of a program, allowing for easier management and access throughout the code. The ball in the game serves as an excellent example of a global variable because it is a central object that likely needs to be accessed and modified by various functions across the game for tasks such as movement, collision detection, and rendering. Storing the ball as a global variable ensures that all parts of the game can consistently reference and manipulate the ball's state without having to pass it around explicitly as a parameter in every function.

In contrast, a for loop counter is typically confined to the scope of the loop and does not need to be accessed outside of it, making it inappropriate as a global variable. The parameter in a callback function generally exists only within the context of that particular call and therefore would not benefit from being global. Lastly, a color variable intended for use within a single function does not require global status, as it is not shared across multiple functions, thus lacking the broader applicability that defines a good global variable.