What defines a computer virus?

A computer virus is defined by its ability to replicate itself when executed, which is the essence of option B. This means that a virus is designed to attach itself to legitimate programs or files and, upon execution of these infected files, the virus can spread to other files or systems. This self-replicating nature allows viruses to proliferate across systems, making them a significant threat to computer security.

In contrast, the other options do not capture the fundamental characteristics of a computer virus. While damaging files is a potential outcome (as mentioned in option A), not all viruses damage files; some merely replicate without causing immediate harm. Utility programs, as mentioned in option C, are designed to perform tasks that enhance system performance, not to replicate or cause harm like a virus does. Lastly, a data file that improves security (option D) does not align with the definition of a virus, since a virus typically compromises security rather than enhances it. Therefore, the defining characteristic of a computer virus is its ability to replicate independently, as highlighted in option B.