Final answer:
The word in CAPITALS in the sentence is an adverb that modifies the verb.
Step-by-step explanation:
The word in CAPITALS in the sentence "The fattest dog growled threateningly and bounded SUDDENLY forwards" is an adverb. Adverbs describe and modify verbs, adjectives, or other adverbs. In this case, the word "SUDDENLY" is modifying the verb "bounded" to indicate that the action happened quickly or unexpectedly.
Learn more about parts of speech