What type of malicious program replicates itself to spread across networks?

A worm is a type of malicious program that is designed to replicate itself and spread across networks independently. Unlike a virus, which requires a host file to attach itself to and relies on user interaction to propagate, a worm exploits vulnerabilities in network protocols or software to distribute itself automatically. This ability to self-replicate and also the capacity to traverse network connections makes worms particularly dangerous as they can lead to widespread infections without any user intervention.

In contrast, a virus attaches itself to a legitimate program and requires user action for distribution, while a Trojan horse disguises itself as legitimate software but does not replicate. Spyware is designed to monitor user activity and gather information without consent but does not self-replicate. Thus, the defining characteristic of a worm is its autonomous ability to replicate and spread across networks, making it the correct answer to the question.