Which of the following is NOT a type of malware?

The choice identifying "Operating System" as not a type of malware is accurate because an operating system is fundamentally a software platform that manages computer hardware and software resources. It provides the environment in which applications run and includes essential functions for running a computer efficiently and securely.

In contrast, the other options—spyware, worm, and Trojan horse—are all classified as types of malware designed to harm, exploit, or otherwise affect a computer's operation negatively. Spyware is software that gathers data from a user without their knowledge, worms are self-replicating malware that spread through networks, and Trojan horses masquerade as legitimate software to deceive users into executing harmful actions. Each of these types of malware operates with malicious intent, unlike an operating system, which is necessary for the functionality of a computer system.