Preposition 9

 I did not see you ____ the party on Saturday. Where were you?

Prepositions are words that indicate relationships with the other words in the sentence. Prepositions of place are used to refer to a place, usually to denote the location of something or someone.

Let's look at the usage of various alternatives in the option as preposition of place:

 on : to refer to placement of something attached / adjacent to something else; to refer to something displayed on something else

 in : means "inside"

 at : to refer to an exact place where something is situated / event

 for : not used as a preposition of place

Since in the sentence, the reference is being made to an exact event that happened at an exact place, "at" is the appropriate preposition here.

Thus, the correct complete sentence is:

I did not see you AT the party on Saturday. Where were you?