In Aura Components, application events are fired by invoking the fire() method on the event instance.
Option D: fire()
Correct.
To fire an application event, you first get the event instance using cmp.getEvent("eventName") and then call fire() on it.
Example:
var appEvent = $A.get("e.namespace:eventName");
appEvent.setParams({ "param1" : value1 });
appEvent.fire();
registerEvent is used in the component markup to declare that the component can fire or handle an event.
It is not a method used to fire an event.
There is no fireEvent() method in the Aura framework.
The correct method to fire an event is fire().
Option C: emit()
Incorrect.
emit() is not a method used in Aura components.
This method is commonly used in other frameworks but not in Aura.
Conclusion:
The method used to fire an Aura application event is fire(), which is option D.
Reference:
Firing Application Events
Incorrect Options:
Option A: registerEvent
Incorrect.
Registering Events
Option B: fireEvent()
Incorrect.