A function in programming is a set of instructions that performs a specific task and can be reused throughout the program. It helps to organize code, making it more modular and easier to maintain. A function typically takes input, processes it, and returns an output. The option 'A block of code designed to perform a particular task, which can be executed when the function is called' is the best answer because it encapsulates the essence of what a function is within programming. The incorrect options describe aspects that may be related to functions (like variables and looping) but do not accurately define the primary purpose and use of a function.