Concept:A synonym is a word that has the same or nearly the same meaning as another word.Explanation:The word "Diligent" means showing steady, careful, and energetic effort.The best synonym is "Hardworking" because it conveys the same idea of consistent effort.The other options are opposites: "Idle" means lazy, "Careless" means not paying attention, and "Indolent" means avoiding work.Answer:D. Hardworking