Answer :
The proposition would be around. The propositions typically go before a noun. We drove around the city late at night seems to be the answer
We drove |around| the city late at night.
A prepositional phrase tells where one noun is in relation to another noun. In this sentence the preposition around tells you where "we" are in relation to the city. A prepositional phrase ends with a noun. None of the other sentences have a noun that follows the word that could be a preposition.