What is a piece of malicious code that is inserted into a host program to cause damage or gain control over a computer system?

The correct choice identifies a virus as a form of malicious code that attaches itself to a host program. A virus is designed to replicate and spread by infecting other programs and systems, often causing damage in the process. When the infected host program is executed, the virus code is also activated, allowing it to perform its harmful actions such as corrupting data, stealing information, or taking control of a computer system.

Viruses often rely on user actions, such as running the infected application, which distinguishes them from other types of malware. Unlike worms, which can spread independently, or Trojans, which masquerade as legitimate software without self-replication, viruses specifically need a host to propagate. This characteristic of needing a host for execution underlines why it is categorized as a piece of malicious code inserted into host programs.