Final answer:
The correct answer is the operating system, which is essential for managing the computer's hardware and software resources, providing a platform for other programs to function.
Step-by-step explanation:
The program that runs all the time on a computer is the operating system. It is a vital component that manages the computer's hardware and software resources and provides common services for computer programs. Unlike a word processor, web browser, or video editing software, which are applications run by users as needed, the operating system starts working when you turn on the computer and continues to run until the computer is turned off. It functions as an intermediary between the user and the computer hardware, making it possible for other programs to operate smoothly. Therefore, the correct answer is C) Operating system.