The bolded word is "after."
Answer: C) preposition.
Explanation: a subordinating conjunction is a word that joins an independent clause with a dependent clause, in the given sentence we only have one clause, so "after" isn't a subordinating conjunction. An adverb is a word that modifies a verb, an adjective or another adverb, which isn't the case neither. A preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause, in the given sentence, the word "after" acts as a preposition.