The word which is an adverb among the group of answer choices is: c. Early
In English language, an adverb can be defined as a word that is used to modify a verb, adjective, or another adverb in a sentence. Some examples are: yesterday, often, promptly, loudly, slowly, quickly, recently, rapidly, brightly, sadly, carefully, early, etc.
In this scenario, an example of an adverb is early.
Early can be used in a sentence to modify a verb (arrive); "Tony arrived early at yesterday's party"
Note: elderly, lonely, and costly are all adjectives which are used to describe or qualify a noun in a sentence.
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