(A) If the request matches the targeting for one of your mediation groups, that group will fill the ad request by serving an ad from one of its included ad sources.
(B) If the request matches more than one of your mediation groups, AdMob will use the group priority to determine which group should fill the ad request. Conflicts between mediation groups should be avoided, if possible.
(C) If the request doesn’t match any of your mediation groups, it will be automatically filled by the AdMob (default) group, instead.
(D) If the request doesn’t match any of your mediation groups, it will be automatically filled by a third party, instead.
This question is from the Check your Knowledge section of the “Get to know mediation” lesson from the “Grow Revenue with AdMob Mediation” unit which is a part of the Get Started with Google AdMob course.