Young people should never forget that_good health is_important part of happiness.
年轻人不应该忘记健康是幸福的重要组成部分

health “健康”,不可数名词,故前不加冠词
part 可数并且important为元音开头 冠词为an

故选D

【英语牛人团】倾情奉献,满意请记得及时采纳哦 谢谢!