The correct answer is: A. receive an integer variable named num from the main() program.
A function that is prototyped as int calculate(int num) can only receive an integer variable named num from the main() program. This is because the function declaration specifies that the function takes one argument, which is an integer variable named num. The function can only access the value of num that is passed to it from the main() program. It cannot access any other integer variables in the main() program.
Option B is incorrect because the function declaration specifies that the function takes one argument, which is an integer variable named num. The function cannot receive any other integer variables from the main() program.
Option C is incorrect because the function declaration specifies that the function takes one argument, which is an integer variable named num. The function cannot receive any other integer variables from the main() program.
Option D is incorrect because the function declaration specifies that the function takes one argument, which is an integer variable named num. The function can receive the value of num that is passed to it from the main() program.