Final answer:
In the grammatical sense, 'carefully' acts as an adverb in the sentence provided because it describes how the action (work) is performed.
Step-by-step explanation:
In the given sentence, the word 'carefully' is a type of word used to modify or qualify a verb, an adjective, or another adverb, hence, 'carefully' is an adverb.
Adverbs often provide more detail to how, when, where, and to what extent something is done. In this case, 'carefully' is detailing how the person is working.
Learn more about Adverbs