I saw you dancing . . . . . . . . party. A. at B. at a C. at the D. in

The correct answer is: D. in

The preposition “in” is used to indicate a location inside a place or thing. In this case, the party is being held inside a building or other enclosed space.

The preposition “at” is used to indicate a location at a specific point in time or space. In this case, the party is happening at a specific time and place.

The preposition “a” is used to indicate a general or unspecified thing. In this case, the party is not a specific party, but rather any party.

The preposition “the” is used to indicate a specific or particular thing. In this case, the party is a specific party that has been mentioned before.