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

A server using a non-standard or unknown port is simply an uncommon service configuration and does not involve misusing a protocol's intended function to covertly transfer information.

A covert channel is technically defined as using a protocol or communication mechanism in an unintended manner to secretly transfer information, such as encoding data inside ICMP echo requests or DNS queries to exfiltrate data past firewalls. This misuse of legitimate protocols violates the security policy of the system without using any officially sanctioned communication path.

Multiplexing is a standard, intentional technique for sharing bandwidth across multiple signals on a communication link and has no relation to covert information transfer.

WEP's insecurity is caused by weak initialization vectors and flawed RC4 key scheduling, not by any concept related to covert channels.