The word "extremely" is an adverb that modifies the adjective "happy".
The word in the sentence that is an adverb that modifies an adjective is "extremely."
The word in the sentence that is an adverb modifying an adjective is "extremely." To determine this, we need to identify the adverb (extremely) and the adjective it is modifying (happy). The adverb "extremely" intensifies the adjective "happy," indicating the degree of happiness Maria felt upon hearing her poem was accepted for publication. Therefore, the correct answer is "extremely."
