The adverb of this sentence is the term “angrily.”
An adverb is a word that describes the verb of a sentence. In this sentence, we are told that Nathan stamped his feet. Therefore, the verb is “stamped.” When thinking of an adverb, a common thought process would be: How did this person do this action?
With that in mind, we need to consider the mannerism of how Nathan stamped his feet. Because we are told he stamped his feet “angrily,” the term “angrily” is the adverb of this sentence.