Which of the following types of channels is used by Trojans for communication?

Loop channel is not a recognized term in networking or malware communication taxonomy and does not describe any method used by Trojans to communicate.

An open channel is a publicly accessible and visible communication path, which Trojans deliberately avoid because the transparency would make detection and blocking trivial.

A covert channel is a communication pathway that exploits system resources or protocols in unintended ways to transfer data without triggering security monitoring. Trojans use covert channels - such as encoding commands inside DNS queries, HTTP headers, or ICMP payloads - to receive instructions and exfiltrate data while appearing as normal traffic. This concealment is fundamental to Trojan design and is what distinguishes their communication from overt, detectable network activity.

An overt channel is a standard, authorized communication pathway operating within normal protocol specifications, making it unsuitable for Trojans that must conceal their traffic from security monitoring.