Final answer:
The prepositional phrase in the sentence provided is 'behind the door,' which describes the location of the subject, 'the young man'.
Step-by-step explanation:
The prepositional phrase in the sentence 'The young man standing behind the door was a fifteen-year-old country lad' is 'behind the door'. A prepositional phrase typically begins with a preposition and includes the object it refers to. In this case, 'behind' is the preposition and 'the door' is the object of the preposition, making the entire phrase function adverbially to describe the location of the young man.
The prepositional phrase in the sentence is behind the door. The prepositional phrase functions as an adverbial and provides information about the location of the young man. It answers the question 'where'.