What is a program called when it secretly attaches to a document or program and executes when the document or program is opened?

The correct answer is that a program which secretly attaches to a document or program and executes when that document or program is opened is known as a computer virus. A computer virus is specifically designed to replicate itself and spread to other files and programs, often causing harm to the host system. It relies on some form of user action, such as opening a file or running a program, to activate and proliferate.

When it infects a document or program, it can go undetected until the user triggers the execution, at which point it can perform various malicious actions, including corrupting files, stealing information, or further disseminating itself to other systems. This characteristic of needing a host and executing upon user interaction is what differentiates it from other forms of malware.

Adware typically displays unwanted advertisements rather than attaching itself to files, and although it can collect user data, it does not spread in the same way a virus does. A Trojan horse disguises itself as legitimate software but does not necessarily replicate like a virus. Spyware is designed to monitor user activity and collect personal information without consent, rather than spreading itself like a virus.