Which of the following is considered the best way to counter packet monitoring for a...

Port mirroring (also known as SPAN - Switched Port Analyzer) is considered one of the best ways to counter packet monitoring on a switch. This technique involves copying traffic from one or more switch ports (or an entire VLAN) to another port where the monitoring device is connected. Port mirroring allows administrators to monitor network traffic in a non-intrusive way, as it does not affect network performance and is transparent to users and endpoints on the network.References:

Cisco Systems, "Catalyst Switched Port Analyzer (SPAN) Configuration Example".