Question:

Which of the following explains why the APs are still vulnerable in the case
You and your customer are talking about wireless security. He claims that his network is secure because he has installed MAC filtering on all access points, allowing only MAC addresses from clients he has manually configured in each list. As you point out, this procedure will not prevent a determined attacker from joining his network.

A An attacker could sniff an existing MAC address and spoof it.

explanation

MAC filtering can readily be circumvented by sniffing the network for a valid MAC and then spoofing it using any of the available alternatives.