Tom still live in the apartment,___ the window faces east.

此题考察定语从句的用法.
where在定语从句中做地点状语,如果选where,它那就相当于 in the apartment 后一句就变成了在公寓中窗子朝东,但在此句中想要表达的意思是 公寓的窗户朝东 apartment 与window 是所属关系,所以用了of which.（which 是关系代词,代替先行词apartment）
实际上后一句也可用这种形式 the window of which faces east 或 whose window faces east (注意：window前没有the)