這一題關係代名詞不懂,能否解釋?

這一題考關係代名詞,能不能請高手詳細說明解題的過程(不要只有翻譯和答案)。謝謝。

The manager nodded, although it was not at all clear ______ he was agreeing.

a. who b. whom c. to whom d. with whom
Update: 請問 it 指什麼?whom又代替那個先行詞?
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