What is a subroutine in programming?

A subroutine in programming refers to a set of instructions that can be defined once and called multiple times from different parts of a program. This functionality allows for code reuse, which simplifies programming and makes the code more organized and easier to maintain. When a subroutine is called, the program execution temporarily jumps to the set of instructions defined in the subroutine, executes them, and then returns to the point where the subroutine was called. This concept is fundamental in programming to promote modularity and reduce redundancy.

In contrast, the other options do not accurately describe a subroutine. A set of instructions executed once pertains more to a one-time execution block, which does not encapsulate the reusable nature of a subroutine. A variable that stores data refers to a storage element in programming but does not encompass the functional aspect of executing instructions. An error handling mechanism, while crucial in programming, specifically deals with managing errors that occur during program execution, which is separate from the purpose and operation of subroutines.