Which sentence uses a prepositional phrase? O Given the economic and political unrest, the family decided to emigrate.
Ruling brutally, the harsh dictator caused many people to flee the country
Increasing tensions required that international diplomats begin to negotiate.
Some editorials exposing government corruption were leaked to the public,

A prepositional phrase is a phrase that starts (or is headed by) a preposition, that is a word like "to", :"in", or similar. The preposition here is "to" - and the prepositional phrase is "to the public". The whole sentence is "Some editorials exposing government corruption were leaked to the public.". (the other instances of "to" are not propositions, but part of the infinitive