The Network Management Station uses SNMP to manage devices.

SNMP Message Types:

get-request: Sent by the NMS to retrieve data from a device.

set-request: Used by the NMS to modify device settings.

get-response: Sent by the device in response to a get-request.

trap: Sent automatically by a device to notify the NMS of an event.

Why "trap" Is Correct:

If a registered event occurs (e.g., interface down, high CPU usage), the device proactively sends a trap to alert the NMS.

Conclusion:

Since traps notify abnormal events, the correct answer is A (trap).

References: HCIA-Datacom V1.0 Certification Guide, Chapter 7: SNMP and Network Monitoring.