While reviewing the wireless router, a systems administrator of a small business determines someone is spoofing the MAC address of an authorized device. Given the table below:

Which of the following should be the administrator's NEXT step to detect if there is a rogue system without
impacting availability?

A. Conduct a ping sweep,
B. Physically check each system.
C. Deny internet access to the "UNKNOWN" hostname.
D. Apply MAC filtering.