What keyword is used in C++ to indicate that a function does not return a value?

The keyword "void" is used in C++ to indicate that a function does not return a value. Options A, C, and D are incorrect because they are either not keywords in C++ or do not accurately describe a function that does not return a value. For example, "null" is not a keyword in C++ and "empty" and "none" do not explicitly indicate that a function does not return a value. Therefore, "void" is the best keyword to use in this scenario.