正确答案是：We live in a beautiful house.there is a river in front of it.
= We live in a beautiful house in front of (which is) a river.
in front of which is a river
= a river is in front of of which
= a river is in front of the house
此定语从句采用了完全倒装语法,即主语为名词（a river）时,可把表语in front of which提前,再加be动词is,再加主语.