The correct answer is: A. such a
The word “such” is used to emphasize the quality or extent of something. In this case, the speaker is emphasizing how wonderful the time they had at the party was.
The word “so” can also be used to emphasize something, but it is more general than “such.” “So” can be used to emphasize the quality, extent, or degree of something. In this case, the speaker could have said “We had so much fun at the party last night,” but that would not have been as specific as saying “We had such a wonderful time at the party last night.”
The word “such” can also be used to introduce a list of things. In this case, the speaker could have said “We had such a wonderful time at the party last night. We danced, we ate, and we laughed.”
The word “very” can be used to emphasize something, but it is not as strong as “such.” “Very” can be used to emphasize the quality, extent, or degree of something. In this case, the speaker could have said “We had a very wonderful time at the party last night,” but that would not have been as specific as saying “We had such a wonderful time at the party last night.”