Which definition among those given below best describes a covert channel?

A server listening on a non-standard or non-well-known port is simply using an uncommon port assignment and does not misuse or repurpose any protocol in an unintended way.

A covert channel exploits a legitimate protocol or resource outside its intended purpose to secretly transmit information - for example, encoding data in IP header fields, DNS query names, or ICMP payloads that are not normally used for data transfer. This definition captures the essential characteristic: repurposing an existing, trusted mechanism to establish a hidden communication path that evades detection by security controls designed around normal usage patterns.

Multiplexing is a standard, intended networking technique for sharing a communication link among multiple channels and is unrelated to hidden or unauthorized communication.

WEP weaknesses refer to cryptographic vulnerabilities in the 802.11b encryption protocol and describe broken security - not a mechanism for creating a hidden communication channel.