Answer: Worm
Explanation: Worm is the malicious software program which acts as the infection in the computer system. It has the ability to replicate itself and spread in the other systems .It is found in those parts that have automatic operation and non-noticeable.
The effect of the worm is slowing down the operations ,disturb the network etc.Other options are incorrect because virus is the malicious program that infects the host, key-loggers are used for monitoring of system and time bomb is the program for releasing the virus in computer network. Thus, the correct option is worm.