Final answer:
The word 'EXTREMELY' in the sentence is an adverb.
Step-by-step explanation:
The particular part of speech represented by the word EXTREMELY in the given sentence is an adverb. Adverbs modify verbs, adjectives, and other adverbs. In this case, the adverb 'extremely' is modifying the adjective 'excited', expressing the degree to which the speaker is excited for the trip to Costa Rica.
Learn more about Part of speech