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

Using a non-standard or 'not well-known' port for a server program is port obscurity, not inherently a covert channel, as the port is still a legitimate communication endpoint.

A covert channel is a communication path that is not designed or intended to transfer information but can be used for that purpose. This often involves manipulating existing protocols or system features outside their normal operating parameters to surreptitiously exfiltrate data or establish hidden communication.

Multiplexing is a standard technique for combining multiple signals or data streams over a shared medium, which is a normal function of communication links, not a covert channel.

WEP's insecurity stems from weak cryptographic algorithms and key management issues, not from 'weak channels' that constitute a covert channel in the general security definition.