Response is the correct choice. A response would essentially be the action that is taken once an alarm has been produced by an IDS, but is not a fundamental component of the alarm.
The following are incorrect answers:
Communications is the component of an alarm that delivers alerts through a variety of channels such as email, pagers, instant messages and so on. An Enunciator is the component of an alarm that uses business logic to compose the content and format of an alert and determine the recipients of that alert. A sensor is a fundamental component of IDS alarms. A sensor detects an event and produces an appropriate notification.
Domain: Access Control
Reference: Official guide to the CISSP CBK. page 203.