Final answer:
The adverb in the sentence 'That snowman is melting so quickly!' is the word 'quickly,' which describes the speed of the snowman's melting.
Step-by-step explanation:
The adverb in the given sentence “That snowman is melting so quickly!” is the word “quickly”. Adverbs often modify verbs by providing more information on how the action is performed. In this case, “quickly” is modifying the verb “melting”, describing the manner in which the snowman is melting.