Which statement correctly describes a function?

A function is fundamentally defined as a block of code designed to perform a specific task and can be reused throughout a program. This reusable nature allows developers to encapsulate a set of operations which can then be invoked multiple times without needing to rewrite the same code, thus promoting DRY (Don't Repeat Yourself) principles and improving maintainability.

In contrast, a single variable declaration pertains solely to defining a variable and does not inherently carry out any tasks or operations like a function does. Similarly, handling errors typically involves mechanisms such as try/catch statements rather than defining a set of executable code, and a loop structure is concerned with repetition of code segments rather than executing a distinct task with modularity as functions do. Therefore, the option highlighting a function as a reusable block of code accurately captures its essential purpose in programming.