Final answer:
To force a switch to fall back to forwarding mode, Harold should use a MAC flooding attack. This type of attack overloads the switch's MAC address table, causing it to enter a fail-open state and act as a hub. Other options like ARP spoofing and sniffing are not designed to induce this specific state on a switch.
Step-by-step explanation:
Harold is performing a penetration test and would like to force a switch to fall back to forwarding mode. To achieve this goal, the most effective attack would be b) MAC flooding. During a MAC flooding attack, an attacker floods the switch with a large number of Ethernet frames, each with a different MAC address. This can overwhelm the switch's MAC address table, causing it to enter a state known as a fail-open mode, wherein the switch starts behaving like a hub by broadcasting all incoming packets to all ports because it cannot determine their intended destination.
ARP spoofing, active sniffing, and passive sniffing are different types of attacks that do not typically trigger a switch to fail-open or revert to forwarding mode. ARP spoofing involves sending forged ARP messages to a local network segment. Active sniffing is when an attacker must inject traffic into the network to uncover hidden networks, and passive sniffing is merely the act of eavesdropping on network traffic without altering it.