核心问题
——What do you think of the boy? ——The boy ____ I believed to be honest fooled me. A. who B. what C. which D. whom
《新高考英语必考语法精析》(商务印书馆国际有限公司, p.279) 对答案 D (whom) 作了如下的解答: “这是形容词从句中加进插入语的用法:
(1) This is the girl who I believe was the driver of that car.
(2) I picked up a man who I thought was honest. = I picked up a man whom I thought to be honest.
[注意]who与whom由于在形容词从句中所处的地位不同，而取不同的形式。”
我认为上题中的 I believed 不像是插入句。您说呢？
词典解答
该书作者所列举的两个例句是正确的，但是对试题的解答却是错误的。试题中的 I believed 并不是插入语，而是定语从句的主语和谓语。whom 是动词 believed 的宾语，to be honest 则是宾语补足语。本题的主句是 The boy ... fooled me.
此外，选项 A(who) 也是正确的，而且更为多见，见 who/whom。