【正确答案】 C

【答案解析】[理由] agree通常作不及物动词，有下列用法：
agree with sb 同意某人
agree to sth 同意某事
[例] I totally agree with you on this issue.
(针对这个议题我完全同意你的看法。)
I'm afraid that Mom won't agree to this marriage.
(恐怕妈妈不会答应这件婚事。)
由于agree是不及物动词，故不可直接接which作agree的宾语，必须在which前加上to方可形成“agree to”的用法，故(C)应为正选。
原句可改写如下：
The proposal, which I agree to, is unfortunately not accepted by my partner, ...
agree也可作及物动词，但须接that所引导的名词从句作宾语。
[例] We all agree that these old-fashioned ideas won't work for youngsters.
(我们都同意这些陈旧的观念，但对年轻人来说并不管用。)