Which changes are necessary for Switch A to meet the requirements?

Port security needs to be enabled on the interface.
Port security needs to be configured to allow only one learned MAC address.

As we see in the output, the Port Security is in Disabled state (line 2 in the output). To enable Port security feature, we must enable it on that interface first with the command: SwitchA(config-if)#switchport port-security -> B is correct. Also from the output, we learn that the switch is allowing 2 devices to connect to it (switchport port-security maximum 2) but the question requires allowing only PC_A to access the network so we need to reduce the maximum number to 1 -> D is correct.