Which of the following parts of speech describes the word 'incredibly' as used in the sentence: 'The pumpkin crop was incredibly small this year because of excessive rain in the spring and heat in the summer'?

In the sentence provided, 'incredibly' is used to modify the adjective 'small,' indicating the degree of smallness. Adverbs can modify verbs, adjectives, or other adverbs. In this context, 'incredibly' functions as an adverb by providing more information on the extent of the adjective 'small.' Therefore, the correct answer is 'adverb.' Adverbs answer questions like 'how,' 'when,' 'where,' 'why,' or 'to what extent' and can modify adjectives to provide more detail or context. In this case, 'incredibly' modifies 'small,' demonstrating the degree of smallness, making it an adverb in this sentence.