The prepositions are the words which show the relationship between the pronoun and the nouns. On the other hand, the prepositional phrase starts with the prepositions and end with the object of a preposition, having one or more modifiers to describe.
Therefore, the correct analysis of the sentence, "n a feeding frenzy, the pigeons descended upon the breadcrumbs that the kindly old man on the park bench continued to toss." is The prepositional phrase “on the park bench” acts as an adjectival phrase.
The prepositional phrase acts as an adjective or adverb when it answers, "which one?"
Hence answering the question "which bench?"- "on the park bench".
