Final answer:
The word 'quietly' is the adverb in the sentence 'I tiptoed quietly downstairs' because it describes the manner in which the action of tiptoeing was carried out.
Step-by-step explanation:
An adverb is a word that modifies a verb, an adjective, or another adverb by giving more information about how an action is performed. In the sentence I tiptoed quietly downstairs, the word quietly serves to describe how the action of tiptoeing was performed, which makes it an adverb. It is used to indicate the manner of the action, so the correct answer is c) quietly.