Which type of malware self-replicates?

The choice of worm as the correct answer is based on its defining characteristic of self-replication. Worms are a type of malware that can reproduce themselves and spread independently across networks without needing to attach themselves to a host file or program. This ability allows them to propagate quickly, often exploiting vulnerabilities in operating systems or software to gain access to new systems.

In contrast, viruses require a host program to attach themselves to and depend on user action to spread, such as opening an infected file or program. Trojans disguise themselves as legitimate software to trick users into executing them but do not self-replicate; instead, they rely on deception for distribution. Rootkits are tools that modify system components to hide their presence and allow unauthorized access but do not have the capacity for self-replication. Therefore, the unique ability of worms to independently replicate and spread makes them the correct answer in this context.