Final answer:
The word 'marshmallow' in the sentence is a noun, not an adjective or an adverb.
Step-by-step explanation:
The word 'marshmallow' in the sentence is used as a noun. It is not an adjective or an adverb. In this sentence, it is referring to the thing that Grunkle Stan Pines is waiting for - the delivery of marshmallows.
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