What is a 'Trojan' in the context of network security?

Correct option is D

A Trojan, or Trojan Horse, is a type of malicious software that pretends to be a legitimate application. Once installed, it can provide unauthorized access to the attacker, often leading to security breaches.
(a) A protocol used for secure communication between devices on a network: This is incorrect because a Trojan is not a protocol; it is malware.
(b) A program that monitors network traffic for security breaches: This is incorrect because a Trojan is designed to infiltrate, not to monitor network traffic.
(c) A type of firewall that protects against viruses and malware: This is incorrect because a Trojan is malware itself, not a protective measure like a firewall.