Which of the following security threats is defined by its self propagating characteristic?

A virus attaches itself to a legitimate host file and requires user action (running the infected file) to spread, so it lacks autonomous self-propagation.

A rootkit is designed to gain and conceal privileged access to a system; its defining trait is stealth and persistence, not self-propagation.

A Trojan disguises itself as legitimate software to trick users into executing it, but it does not self-replicate or spread on its own.

Unlike other malware types, a worm copies itself and propagates independently across networks by exploiting vulnerabilities or network shares. It does not need to attach to an existing file or rely on a user to execute it, making self-propagation its defining characteristic.