There are more than 50 proposals ___at the conference.

这里是to引导的介词短语做proposals的后置定语,而这里显然不是主动的动作,所以又用了被动语态.至于D嘛,是现在完成进行时,不适合用在这种语境,意思和时态也不对.
现在完成进行时
其构成形式如下：
I / we / they have
He / she / it has been + 动词的现在分词
1） 表示一个在过去开始而在最近刚刚结束的行动,如：
Ann is very tired.She has been working hard.
Why are you clothes so dirty?What have you been doing?
2) 表示一个从过去开始但仍在进行的行动,如：
It has been raining for two hours.(现在还在下)
Jack hasn’t been feeling very well recently.
3) 表示一个从过去开始延续到现在,可以包括现在在内的一个阶段内,重复发生的行动,如：
She has been playing tennis since she was eight.
4) 现在完成时强调动作行为的结果、影响,而现在完成进行时只强调动作行为本身,如：
Tom’s hands are very dirty.He has been repairing the car.
The car is going again now.Tom has repaired it.