Which of the following is a covert channel type?

Storage covert channels work by having one process write data to a shared storage location (such as a file, registry key, or attribute) that another process can then read, thereby communicating information outside of authorized channels. This is one of the two canonical covert channel types defined in security literature (storage and timing), originating from the Lampson covert channel model. The storage channel exploits shared resources to leak information without direct communication.

A pipe is an inter-process communication (IPC) mechanism, not a recognized category of covert channel; while pipes could theoretically be misused, 'pipe' is not a defined covert channel type in security frameworks.

Memory is a resource that may be leveraged in a storage covert channel, but 'memory' itself is not a named covert channel type; it is a component that could be part of a storage-based covert channel.

Monitoring is a defensive security activity used to detect threats and anomalies on a network or system, and has no classification as a covert channel type.