What is a reference parameter in a subprogram definition?

A reference parameter in a subprogram definition allows the function or procedure to modify the value of the argument passed to it directly. When a reference parameter is used, the function can access and change the value of the variable that was passed in. This means that any changes made to the parameter within the subprogram will affect the original variable outside of the subprogram, reflecting the changes when control returns to the caller.

This behavior distinguishes reference parameters from parameters passed by value, where a copy of the variable is made and any changes are local to the subprogram. Therefore, when using reference parameters, it is crucial to understand that the original data can be altered, enabling the subprogram to return updated values to the caller without needing to pass return values explicitly.

Other choices do not accurately represent what a reference parameter is and how it operates within a subprogram context. For instance, a variable that cannot be modified would suggest immutability, which contrasts with the very purpose of a reference parameter, while a default parameter refers to a parameter that has a predefined value if none is provided by the caller.