The correct answer: A remote file access or transfer service to another system to copy itself from one system to the other.
A worm is a program that actively seeks out more machines to infect, then each infected machine serves as an automated launching pad for attack on other machines.
A worm can replicate itself through electronic mail or instant messaging, file sharing, remote execution capability, remote file access or transfer capability, remote login capability.
A worm executes a copy of itself on another system, either by using an explicit remote execution facility or by exploiting a program flaw in a network service to subvert its operations.