After several minutes I finally located my keys.
"After several minutes" is a prepositional phrase (option A). It is formed by two parts: the preposition plus a noun (the object of the preposition). In this example "after" is the preposition and the word "minutes" functions as the object of the preposition. Between the preposition and the object in prepositional phrase, it is possible to find out an adjective; that's why the word "several" is used as an adjective which describes and modifies the noun.