Concept:The correct phrasal verb must show that the celebrations ended in a suitable way.Explanation:The phrasal verb "rounded off" means to bring an activity to a clear or satisfactory conclusion.In this sentence, the firework display was the final event of the celebrations.The other options do not fit: "round up" means to gather, "round out" means to complete in shape or content, and "round down" means to reduce a number.Therefore, "off" is the best choice.Answer:C. off