What is the purpose of defining a function in programming?

Defining a function in programming serves the fundamental purpose of encapsulating a specific task or operation that can be executed whenever it is called. Functions allow programmers to write reusable and modular code, which enhances readability and maintainability. When a function is defined, it can take parameters, perform computations or actions, and return a result. This modular approach means that you can write a piece of code once and use it in multiple places within a program without the need to rewrite the same logic each time.

By using functions, developers can break down complex problems into simpler, manageable pieces, improving the overall structure of the code. This also facilitates easier debugging and testing, as functions can be tested in isolation.

The other options, while relevant to programming, do not accurately capture the core purpose of a function. Functions do not inherently relate to user interfaces, executing loop iterations, or storing multiple variables, as these tasks can be accomplished through other constructs and paradigms in programming. Functions are specifically designed to perform well-defined tasks when invoked, making them a key construct in programming languages.