As it turned out to be a small house party,we ___ so formally.

As it turned out to be a small house party,we ___ so formally.
答案：needn't have dressed up
didn't need have dressed up 和 didn't need dress up 肯定错误。因为有didn't ,need 肯定是实义动词，后面必须加to.
needn't dress up 时态和语气都不符合，
needn't have done 表与过去事实相反的虚拟语气，意思“本不必做”，“as it turned out to be a small house party,we needn't have dressed up so formally.”意思“由于（最终/结果）那只是一个小小聚会，我们本不必如此正式打扮.”needn't dress up 时态和语气都符合 。
，。

由于这只是一个小小的家庭聚会,我们本来不需要穿的这么正式.

选needn't have dressed up
本来不需要打扮，表与事实相反。

as it turned out

turn out 事实证明；最终是...

我选C
个人觉得还是用过去式，由于那只是一个小聚会，我没就没有穿的这么正式。
B和D好象没有见过这种用法，NEED后面根本就不可以接完成时（我好象没见过）如果选B那D也可以选，请参考