Final answer:
In the provided sentence, 'off' functions as an adverb, providing more information about the verb 'fell' and the way the muffler fell.
Step-by-step explanation:
In the sentence "The muffler just fell off," the word "off" serves as an adverb. It modifies the verb "fell" by describing where the muffler fell. Prepositions typically require an object, nouns serve as a subject or object, and conjunctions connect clauses or sentences. Since "off" here is not fulfilling any of these roles and is instead telling us more about how the muffler fell, it is categorized as an adverb. Therefore, the correct answer is B) Adverb.