i'd like to buy a book,( )that was written by LuXuni'd like to buy a book,( )that was written by LuXunawhich b one cbut d all应该选b谁告诉我为什么选B不选A啊

B
这里的不定代词ione是指代book，作同位语，one后面来一个定语从句，这个定语从句用that引导，修饰这个one。希望我能帮助你解疑释惑。

因为有that了。
如果用which应该是不加逗号
i'd like to buy a book which was written by LuXun

B
这里的不定代词ione是指代book,作同位语,one后面来一个定语从句,这个定语从句用that引导,修饰这个one.希望我能帮助你解疑释惑.

逗号后面是一个完成的句子，that was written by LuXun，有主谓宾，不缺成分。
用which引导的话，which要充当主语的成分。