また文法で質問です。

It has long been accepted that Beowulf is one of the most significant epic works in English, but it is still unclear
______ the poem was written.

(a)by whom
(b)for which
(c)of
(d)with

質問１．解答は(a)なのですが、(b)がありえない理由がわかりません。

質問は以上です。よろしくお願いします。