Final answer:
The prepositional phrase in the provided sentence is 'Before Louisa May Alcott published Little Women,' and it acts as an adverb setting the time frame relative to the main action in the sentence.
Step-by-step explanation:
The prepositional phrase in the sentence “Before Louisa May Alcott published Little Women, which would launch her into literary history, she published several works using the pseudonyms Flora Fairfield and A. M. Barnard.” is “Before Louisa May Alcott published Little Women.” A prepositional phrase includes a preposition (in this case, “before”) and its object (here, “Louisa May Alcott published Little Women”), which can be a noun, pronoun, gerund, or clause. This phrase acts as an adverb, providing context by indicating a time frame relative to the main action of publishing works under pseudonyms.