1.Look at that handsome boy (C whose) name is John.
the boy's name is John,用boy的所有格,“他的名字”,所以选whose
2 Would you like to give me the picture-book (C whose) cover is white?
理由同上,the picture-book's cover is white,用picture-book的所有格,“图画书的封面”,所以选whose
3.Have you found the information about famous people (A which) you can use for the report.
这里是修饰名词information的定语从句,因此要选which.who,whom是修饰人的,排除,what是用来引导主语/宾语从句的,不能引导定语从句.