When changing from active to passive voice, the subject becomes the object and the sentence in passive voice changes to "WH + be + S + V3 + prep + object".
So, here the object "my proposal" becomes the subject and the subject "you" becomes the object.
Thus, the correct passive voice of the given sentence is option D "Why was my proposal not agreed to by you". It follows the rule "Why + was (be) + my proposal (object in active becomes subject in passive) + not agreed (past participle of verb not agree) + to (preposition) + by you (subject becomes the object).
Option A is incorrect as it is missing the object.
Option B is incorrect as it is missing the preposition.
Option C is incorrect as the auxiliary form of 'be' (was) precedes the subject in a 'WH interrogative sentences', it does not follow it. 
Therefore, option D is the correct answer.