Explanation:

System software is a type of computer program that is designed to run a computer's hardware and application programs. The **Operating System (OS)** is the most crucial component of system software. It manages computer hardware and software resources, provides common services for computer programs, and acts as an interface between the user and the hardware. Without an OS, a computer cannot function effectively. While compilers and interpreters (options a and b) are also types of system software (specifically language processors), they are not the *main part* or the core foundation upon which all other software runs. A 'package' (option d) is a collection of software, which might include application software or system utilities, but it's not the fundamental system software itself. Thus, the operating system is unequivocally the main part.