Concept:The correct phrasal verb is “hit on”, which means to suddenly think of or discover an idea.Explanation:The sentence says he finally found the idea of travelling abroad.The phrasal verb “hit on” means to arrive at or think of an idea.“Hit out” means to attack; “hit at” means to aim a blow; “hit up” means to approach or request.Thus, only option C, “on”, fits the meaning correctly.Answer:C. on