The correct answer is D. multitasking.
Multitasking is the ability of a computer system to run multiple programs at the same time. This is done by dividing the processor time between the programs. The operating system is responsible for scheduling the programs and ensuring that they all have a fair share of the processor time.
Word processing is a type of application program that allows users to create and edit documents. Compiling is the process of translating a program written in a high-level language into a low-level language that can be executed by the computer. Virtual memory is a technique that allows a computer to use more memory than is physically installed on the system.
None of these options are required for a system program to execute more than one program at a time.