Rationale
Swiftly is the adverb in the sentence.
An adverb modifies verbs, adjectives, or other adverbs, often indicating manner, time, place, or degree. In this case, "swiftly" describes how an action is performed, making it the only adverb among the given choices.
A) "Ran" is a verb that indicates an action but does not describe how, when, or where that action takes place. It simply represents the activity of running and therefore does not function as an adverb in the sentence.
B) "Athletic" is an adjective used to describe a noun, typically conveying qualities related to physical fitness or sports. As it modifies a noun rather than providing additional information about a verb or another adjective, it cannot be classified as an adverb.
C) "Across" is a preposition that typically indicates a position or direction. It serves to link nouns to other words in the sentence—specifying spatial relationships but not modifying any verbs or adjectives. Thus, it fails to fulfill the function of an adverb.
D) "Swiftly" effectively modifies a verb by indicating the manner of the action being performed. It describes how something is done, making it an adverb. This choice directly addresses the question by fulfilling the role of an adverb in the sentence.
Conclusion
Identifying adverbs is crucial for understanding sentence structure and meaning. Among the options provided, only "swiftly" serves to modify an action, clearly fulfilling the role of an adverb. The other choices—"ran," "athletic," and "across"—either represent different parts of speech or fail to modify the action as required by the definition of an adverb. This distinction underpins effective communication and clarity in writing.