The correct answer is C) determine if he is breathing. When a person is unconscious from electric shock, the primary action is to remove them from the electrical source to stop the flow of electricity through the body. Once this is done, the secondary action is to check if the person is breathing, as electric shock can cause respiratory arrest. Determining if the person is breathing is crucial to assess if they require rescue breathing or CPR. The other options are incorrect because checking for burns (A) or massaging vigorously (B) should not be the immediate priority, and administering ammonia smelling salts (D) is not the appropriate first aid measure for an unconscious person.