What is a function in programming?

A function in programming is fundamentally a block of code designed to perform a specific task, and it often has the capability to return a value to the part of the program that called it. This ability to return a value is what sets functions apart from other programming constructs such as procedures. A function can take inputs, known as parameters, execute the defined operations, and then produce an output, which enhances code reusability and modularity.

In contrast, other options describe different concepts in programming. A block of code that cannot return a value refers more closely to a type of procedure or subroutine but does not encompass the full definition of a function. Similarly, a variable that holds constant values pertains to data storage rather than functional behavior. Lastly, an outline of processing steps does not convey the specificity and return capability that defines a function. Therefore, a programming function is best characterized as a procedure that performs a task while having the potential to return a value, making option B the correct choice.