Insurmountable (adjective): impossible to exceed. Near the beginning of the story, the author writes, "In the living room sat a magnificent mini bike. It was red with a pearly white gas tank and side panels. For a brief moment, Lily was filled with what seemed like insurmountable excitement. She couldn’t imagine anything better than getting that bike.” Since getting the bike is the best thing Lily can imagine, we can understand that her excitement is the highest it could be. Insurmountable means impossible to exceed. Surpassable means capable of being exceeded. Since this is the opposite of being impossible to exceed, surpassable is an antonym for insurmountable. Therefore (D) is correct.