is a device infected by malware, which becomes part of a network of infected devices administered by a single attacker or attack group.

The correct answer is "botnet".  

Explanation:

A botnet is the interconnection of devices in which each of the devices is operating in more than one bots. The main objective of botnet is used to attacking and stealing  the information, .The botnet is used to operate the illegal task . A botnet is infected by the malware that becomes a part of the network of the infected devices which is operated by a single attacker or attack group.