A set of unwanted instructions executed on a computer and resulting in a variety of...

A virus is a type of malicious software that attaches itself to legitimate files and spreads when those files are executed.

A worm is a standalone malware program that replicates itself to spread to other computers without needing to attach to a host file.

A bug refers to an error or flaw in software, but in the context of this question (malicious instructions causing effects), it can contribute to vulnerabilities exploited by malware.

Since all these can involve sets of unwanted instructions leading to harmful effects, “All of the above” is the most accurate answer.