Question

I'll give you my friend's address,(where) I can be reached most evenings.

Answer 1

这里的连接代语是跟上一句的宾语的,上一句的宾语是my
friend's
address,我朋友的地址,是指一个地方,所以用where

Answer 2

首先这句话是一个非限制性定语从句，
I
can
be
reached
most
evenings
这句话是完整的，就是主谓宾都有，所以
where
是来修饰
前面的先行词
my
friend's
address,
在
I
can
be
reached
most
evenings
中做状语。