Final answer:
The adverb 'cheerfully' modifies the verb 'whistled,' and the phrase 'for Heather's birthday party' is an adjective phrase that modifies the noun 'cupcakes,' so the correct choice is B.
Step-by-step explanation:
In the sentence, "Melissa whistled cheerfully as she frosted the cupcakes for Heather's birthday party," the underlined word 'cheerfully' is an adverb because it modifies the verb 'whistled,' indicating the manner in which Melissa whistled. The adverb 'cheerfully' modifies the verb 'whistled,' and the phrase 'for Heather's birthday party' is an adjective phrase that modifies the noun 'cupcakes,' so the correct choice is B.
The phrase 'for Heather's birthday party' acts as an adjective phrase, as it modifies the noun 'cupcakes' by specifying for what occasion the cupcakes are being frosted. Therefore, the correct option is B. Cheerfully: adverb modifying whistled; for Heather's birthday party: adjective phrase modifying cupcakes.