Answer:
D: growing
Step-by-step explanation:
The word that completes the sentence with a verb functioning as an adjective is growing. The verb grow plus the ing particle form the present participle (used to form continuous or progressive). In "growing season", the same verb has the function of an adjective.
Summer is a noun
Rainy is an adjective but it is not a verb.
Humid is an adjective but it is not a verb.