In which sentence is the underlined prepositional phrase an adjective phrase?

in college football, a tied game ends with no overtime.

a football game has four quarters of 15 minutes.

football games are supervised by neutral referees.

the players run onto the field to start the game.

Adjectival phrases modify nouns, which means that they usually stand next to a noun which they are describing. Having this in mind, the correct answer is B. A football game has four quarters of 15 minutes.
The prepositional phrase here is of 15 minutes, but it functions as an adjective phrase describing the noun quarters. 
A is incorrect because the prepositional phrase acts as an adverbial phrase modifying the verb ends; C is incorrect because the prepositional phrase functions as an adverbial phrase modifying the verb supervised; D is incorrect because of the same reason - the phrase modifies the verb run.