Final answer:
The word 'swiftly' in the provided sentence functions as an adverb, modifying a verb by describing how an action is performed.
Step-by-step explanation:
In the sentence provided, the word that functions as an adverb is 'swiftly.' Adverbs typically modify verbs, adjectives, or other adverbs, providing more information such as how, when, where, and to what extent something is done or happens. In contrast, 'big', 'quick', and 'muscular' are adjectives, which describe nouns. It's important to note that while many adverbs end with '-ly', this isn't always the case, as shown by 'quick', which is an adjective that could become an adverb if changed to 'quickly'. Here are two brief examples to illustrate adverbs and adjectives:
- Example of an adverb: Pierre quickly ran through the main points of his argument.
- Example of an adjective: The big, muscular athlete lifted weights.
So, in the context provided, 'swiftly' is the adverb because it indicates how an action is performed, often answering the question 'How?'.