Which best defines a function in programming?

A function in programming is best defined as a modular segment of code designed to perform a specific task. This definition highlights several key aspects of functions. Functions encapsulate a block of code that can be reused throughout a program, promoting efficiency and reducing redundancy. By defining a function, you can execute its code whenever needed without rewriting it, enhancing both organization and readability.

Functions may also take inputs, known as parameters, and can return outputs, which allows for a flexible and dynamic approach to coding. This modularity helps in breaking down complex problems into smaller, manageable pieces, making it easier to develop, debug, and maintain code.

The other options do not adequately capture the essence of a function. While a collection of related statements that execute automatically suggests some behavior of functions, it does not emphasize the aspect of modularity and specific task performance. A fixed set of variables within a program does not define a function, as functions can have varying and dynamic inputs. Lastly, storing user input is not a characteristic of functions, since functions can process data rather than just store it.