What authority is used when local civilian agencies request military support during emergencies?

Immediate Response Authority is the correct choice because it is a specific legal authority that allows military forces to provide support to civil authorities during emergencies without requiring prior approval from higher authorities. This authority is particularly relevant in situations where rapid action is necessary to save lives, protect property, or prevent great damage during local emergencies. It enables military responders to act swiftly, ensuring that assistance is provided promptly to local civilian agencies when they request help in crisis situations.

The other options, while they sound relevant, do not accurately describe the terminology or practices that govern military support to civilian agencies in emergency scenarios. Emergency Response Authority and Assistance Control Authority are not established concepts within military doctrine concerning support to civil agencies, whereas Operational Support Authority does not specifically apply to the immediate provision of assistance in urgent circumstances as dictated by Immediate Response Authority.