A good friend of mine from ___ I was born showed up at my home right before I left for Beijing.

用WHERE可以。但是句意就变成了，从我家乡来的朋友了（来自我出生的地方的朋友）
选C。一个从我出生起就和我结识的朋友，在我出发去北京前出现在我家里。
A good friend of mine 我的一个朋友。是主语
from _when__ I was born 定语从句，修饰a friend。所以翻译成，从我出生起就结交的朋友。
showed up出现。这句话的谓语。

at my home。这句话的地点状语
right before I left for Beijing就在我出发去北京之前。这句话的时间状语

选C；
A good friend （of mine from when I was born） showed up（ at my home ）（right before I left for Beijing）.
主干A good friend showed up；
括号一修饰friend；
括号二方位状语；
括号三时间状语

CA good friend of mine
from when I was born是借词短语修饰friend，整个句子的骨干是A friend showed up

C
A good friend of mine 是主语 showed up 是谓语
from when I was born 是一时间状语 从我出生的时候

虽说句子有点读不懂，但是语法知识来说选d ,from which=when

C