Final answer:
The preposition in the given sentence is 'along', indicating the boy's relationship to the crowded sidewalk as he hurries.
Step-by-step explanation:
The preposition in the sentence 'A young boy is hurrying along the crowded sidewalk' is the word 'along'. A preposition is a word that indicates the relationship between a noun or pronoun and other parts of the sentence. In this case, 'along' shows the relationship between the action of the boy hurrying and his location, which is the crowded sidewalk.