Which of the following types of malware does not replicate itself but can spread only when the circumstances are beneficial?

A mass mailer actively replicates by sending copies of itself to addresses found in the infected system's contact list, which is a form of self-propagation.

A worm is defined specifically by its ability to self-replicate and spread across networks automatically without requiring user interaction.

A blended threat combines characteristics of multiple malware types - such as viruses, worms, and Trojans - and typically includes self-replication as part of its combined attack vector.

A Trojan horse disguises itself as legitimate software and contains no self-replication mechanism. It can only spread when a user is deceived into executing it or when an attacker manually deploys it under favorable circumstances, such as a user downloading it from a malicious site.