Setting up a packet capture on an Aruba Mobility Controller (MC) is particularly useful in scenarios where detailed analysis of network traffic is necessary to identify and address security concerns. Option B is the correct answer because it directly addresses the need to closely examine the traffic of a potentially malicious wireless endpoint. Packet capture on the MC allows the security team to collect and analyze traffic to/from specific endpoints in real-time, providing valuable insights into the nature of the traffic and potentially identifying harmful activities. This capability is essential for forensics and troubleshooting security incidents, enabling administrators to respond effectively to threats.