Here's why:
* Adjectives modify nouns or pronouns. In this case, "aware" modifies the implied noun "they" or "he" or "she" – "they are keenly aware."
* Adverbs modify verbs, adjectives, or other adverbs. While "keenly" is an adverb, it modifies the adjective "aware," not the noun "they."
Therefore, "aware" in the phrase "keenly aware" functions as an adjective, describing the degree of awareness.