What is a computer virus?

A computer virus is defined as a potentially damaging program that can modify or disrupt the normal operations of a computer system. When executed, a virus can replicate itself, attach to files, and spread from one computer to another, often leading to various issues such as data loss, corruption, or even compromise of system security. This definition aligns with the chosen answer, highlighting the virus's ability to negatively impact a computer's performance and functionality.

The other options do not accurately represent the characteristics of a computer virus. A harmless software application doesn't pose threats or alter performance negatively. A program designed to enhance performance focuses on improving a computer's efficiency rather than risking damage. An update that improves security is intended to protect a system from vulnerabilities, contrasting with the harmful nature of a virus. Thus, the chosen answer encapsulates the essential attributes of what constitutes a computer virus.