Concept:The word "Conundrum" refers to a confusing or difficult problem. Its synonym is a word that shares this same meaning.Explanation:First, understand the meaning of "Conundrum". It means a tricky or puzzling problem. For example: The conundrum of traffic in big cities needs smart solutions.Now, look at the options. "Problem" means a difficult situation to solve. It matches the meaning of conundrum closely.The other options do not match. "Triumph" means a great victory. "Celebration" means a festive event. "Harmony" means peace or agreement.Thus, the correct synonym is "Problem".Answer:D. Problem