Which of the following best describes the concept of callback functions in JavaScript?

Callback functions in JavaScript are a fundamental concept tied to asynchronous programming. They are functions that are passed as arguments to other functions and are executed after the completion of a specific operation or event. This means that when an asynchronous task finishes, it can trigger a callback function to continue the execution flow.

In practical terms, when you perform actions such as reading a file, making a network request, or other operations that may take time, using callback functions allows the program to remain responsive. For instance, a web application can request data from a server and, upon receiving the response, execute the callback function to process that data.

The other options provide incomplete contexts or characteristics of functions rather than capturing the essence of callback functions. Some functions may handle server responses or errors, but that does not encapsulate what makes a callback function unique; it simply describes specific use cases. Declaring functions within another function describes nesting or scope rather than the lifecycle aspect of callbacks, which centers on timing and execution order.