Which of the following best describes a 'function' in programming?

A 'function' in programming is best described as a block of reusable code that performs a specific task. Functions allow programmers to encapsulate a sequence of operations under a single name, which can then be invoked whenever needed throughout the program. This not only promotes code reusability but also enhances maintainability and readability, as the same functionality can be accessed with a simple function call instead of writing out the same code multiple times.

In contrast, a single-use code block, while it may perform a specific task, does not convey the same reusability that is a hallmark of functions. A type of data structure pertains to the way data is organized, such as arrays or lists, which is distinct from the concept of a function. Lastly, while debugging tools are essential in programming for identifying and fixing errors, they do not fit the definition of what a function fundamentally is. Therefore, the focus on reusability and task performance aligns perfectly with what a function represents, making this the most accurate description.