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 <em>of 15 minutes, </em>but it functions as an adjective phrase describing the noun <em>quarters</em>. A is incorrect because the prepositional phrase acts as an adverbial phrase modifying the verb <em>ends; </em>C is incorrect because the prepositional phrase functions as an adverbial phrase modifying the verb <em>supervised; </em>D is incorrect because of the same reason - the phrase modifies the verb <em>run.</em>
