Which of the following types of malware can replicate itself across networks without any user action?

A worm is a type of malware specifically designed to replicate itself and spread across networks autonomously, without requiring any user interaction. Unlike other types of malware, such as viruses, which need to attach themselves to legitimate programs and rely on user action for execution, worms are capable of exploiting vulnerabilities in networked systems to propagate themselves. They can spread rapidly, often consuming bandwidth and resources, which can lead to network congestion and other operational issues.

Considering other types of malware, Trojan horses are disguised as legitimate software but do not replicate themselves; they deceive users into installing them. Viruses need a host program to activate and spread, as they embed themselves in other executable files. Adware is primarily designed to serve advertisements, and while it can be intrusive, it does not replicate itself in the same manner as worms. Thus, worms are distinct in their ability to self-replicate and spread across networks without any action from the user.