The correct option is <em>prepositional phrase</em>
Explanation:
A prepositional phrase is commonly defined as a series of words constituted by a preposition and a noun or pronoun which represents its object.
Bearing the above in mind it could be said that in the sentence <em>"All three newspapers require paid, online subscriptions for people to read content",</em> "for people" correspond to a prepositional phrase, since <em>for</em> is a preposition and <em>people</em> is a noun that represents its object.