What best describes a virus in computer terms?

A virus in computer terms is best described as a type of malware that attaches itself to another program and has the capability to replicate. When executed, the virus can spread to other programs or files, often without the user's knowledge, by modifying those programs and infecting new hosts. This characteristic of attaching itself to existing software and being able to create copies defines the nature of a virus distinctly within the broader category of malware.

The other options do not accurately capture all facets of what constitutes a virus. While some may describe aspects of malware, they either focus solely on the replication aspect or mischaracterize the nature and behavior of viruses. For instance, malware requiring user activation does not encompass the automatic replication feature of a virus, and merely stating that it affects system performance overlooks the replication and attachment behavior that is integral to viruses. Additionally, suggesting that a virus is designed to enhance system security fundamentally misrepresents its intended purpose, as viruses are harmful rather than protective.