He has lost the key to the drawer ____ the papers are kept.

只能填IN WHICH

两个都可以

用in which，把句子还原。
He has lost the key to the drawer.
The papers are kept in the drawer.

用Where，它是代指前面的the drawer.

in which.

which代指drawer.
整句等同于
He has lost the key to the drawer, and the papers are kept in the drawer.

2个答案都对哦。
这是典型的状语从句的用法。
而且是地点壮语！！！

两个都可以。