Choose the sentence that contains an adjectival prepositional phrase. a. the tired campers rested underneath the tent. b. the game begins within an hour. c. the shoe store across the street recently opened. d. the dog ran around the table.

The correct answer should be C The shoe store across the street recently opened. 

An adjectival prepositional phrase" describes a noun or pronoun" I might answer the questions" which one, what kind, how much, or how many." 
in the case "across the street" would function as the adjectival phrase, modifying the noun "store". 

Bis the answer hope this