Question

Prepositional phrases always begin with a preposition and end with

Answer 1

Prepositional phrases always begin with a preposition and end with a noun or a pronoun. At the minimum, a prepositional phrase will begin with a preposition and end with a noun, pronoun, gerund, or clause, the "object" of the preposition. 

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Answer 2

object of the preposition

This is just another name for a noun or pronoun after a preposition.