What causes a port to be placed in the err-disabled state? (CCNA 200-301)

Correct answer: B. link flapping.
A port is placed in the err-disabled state when it experiences a problem that prevents it from functioning correctly. One common reason for this is link flapping, which occurs when the link state of the port is rapidly changing between up and down. This can be caused by a faulty cable, a malfunctioning network device, or other issues.

Link flapping can cause network instability and disrupt traffic flow, so the switch or router will disable the port to prevent further problems. The err-disabled state is a safety mechanism to prevent the port from causing further issues on the network.

The other options are not correct: an empty port (A) will simply be in an "up" or "down" state, but not err-disabled; latency (C) is a measure of delay in network traffic, but it does not cause a port to be err-disabled; and a shutdown command (D) will simply administratively shut down the port, but it will not be in an err-disabled state.