The right answer is A. Trap . In SNMP, a trap is an unsolicited notification sent from a managed device to the management system when a significant event happens. A switch detecting that an interface has gone up or down is a textbook example of the kind of event that can trigger this kind of message.
That is different from normal SNMP polling, where the monitoring server asks the device for status information on a schedule. With a trap, the device does not wait to be asked. It sends the alert immediately, which makes it useful for link failures, threshold violations, and other time-sensitive conditions.
The other choices are related to SNMP, but not in the same way. An object identifier (OID) points to a specific managed value. A string is commonly associated with SNMP community information in older versions. A MIB is the structured database of manageable objects. None of those are the actual alerting mechanism.
Because the question asks for an automatic alert when link status changes, the feature being described is clearly an SNMP trap .