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
Explanation: The author uses third-person omniscient in the article "Klondike Gold Rush" to give the reader information in chronological order and to help the reader understand what was going on during the gold rush and what challenges the miners faced