Final answer:
The word 'extremely' is an adverb in the sentence.
Step-by-step explanation:
The word 'extremely' in the sentence is an adverb. Adverbs modify verbs, adjectives, or other adverbs and often end in -ly. In this case, 'extremely' is modifying the adjective 'courageous' and indicating the degree of courage Helen Keller possessed. It answers the question 'to what extent was she courageous?'
Learn more about Parts of speech