Using a passive network tap is the correct answer because it allows you to monitor network traffic without altering the traffic flow or injecting new traffic, which could be detected by an intrusion detection system. A network tap provides a way to access the data flowing across a computer network. In a passive mode, it is less likely to interfere with normal network operations or to be discovered. Using port mirroring would also avoid detection, but it requires access to switch configuration which may not be available. Ethernet sniffing on a switched network without ARP spoofing is unlikely to capture all traffic due to switch port security. Plugging into an open network port may not necessarily capture all traffic and could potentially be detected if the network employs security measures on unused ports.