从句时态语态有点问题。
I'm rescuing a tortoise that/which is stuck up in a tree.
用which也可以,因为tortoise是物,不能用who。
省略that是不行的,因为that在定语从句中作主语。
定语从句引导词在从句中作宾语,且是限制性定语从句时,可以省略。
如:
This is the book (that/which) I am looking for.
This is the book which/that was given to me.
译:
我在救一只卡在树上的乌龟。
I'm rescuing a tortoise that stuck up a tree. 这个从句除了that还能用which
因为先行词不是人
肯定是which了,人才用who