Final answer:
The swap file serves as virtual memory for a computer when it runs low on RAM, temporarily storing data on the hard drive.
Step-by-step explanation:
The swap file on a computer system is best described as a system file containing the computer's virtual memory. Specifically, when a computer is running low on RAM (Random Access Memory), it uses the swap file to temporarily store data that would normally reside in the RAM.
This file is on the hard drive and enables the computer to use more memory than the physical RAM installed on the system. Therefore, the correct answer to the student's question is B. A system file containing the computer's virtual memory.