Which parameter type allows a function to modify the value of a passed variable?

The parameter type that allows a function to modify the value of a passed variable is known as a reference parameter. When a function uses reference parameters, it receives a reference to the original variable rather than a copy of its value. This means that any modifications made to the variable within the function directly affect the original variable outside the function.

For example, if a function takes a variable as a reference parameter and changes its value, the change will be visible to the caller of the function because both the caller and the function are operating on the same piece of data. This is particularly useful when one wants to return multiple results from a function or when dealing with large data structures where making a copy would be inefficient.

In contrast, input parameters typically provide a copy of the original data to the function, so any changes made to these parameters have no effect on the original variable. Constant parameters are designed to ensure the value cannot be altered within the function, thus preventing any changes to the variable. Output parameters are used to return values from a function, but they do not inherently allow for modifying an existing variable unless they are passed by reference.