Answer:
The correct option is D. as an adverb
Explanation:
Adverbs are words that describe how, where, when and the frequency in which a given action takes place. In other words, adverbs modify verbs by indicating where, when, how or how often, something is being done, reason why they can be identified adverbs of place, adverbs of time, adverbs of manner, among others, which are often placed after the verb that they are modifying.
In the particular case of the example provided "I write furiously" the word furiously correspond to an adverb of manner since it is describing how the action, writing specifically, is taking place.