【正确答案】 D

【答案解析】[解析] 画线词subject为名词，意为“主题”。A项question意为“问题”；例：Answer my questions without hesitation.要毫不犹豫地回答我的问题。B项problem意为“难题，习题”；例：A new problem has been reared up in production.生产中出现了一个新问题。C项title意为“标题，头衔”；例：The title of the novel is Sons and Lovers.这本小说的书名为《儿子与情人》。D项topic意为“主题，话题”；例：The topic has been studied by many investigators.这个课题已经被很多研究者探索过了。故本题选D。