Final answer:
The phrase 'of a sunset' serves as an appositive with nonessential information, adding detail about what is depicted in the picture.
Step-by-step explanation:
The phrase 'of a sunset' in the sentence 'The preschool teacher was shocked that a four-year-old child drew such an exquisite picture of a sunset' functions as an appositive with nonessential information. This phrase adds detail to the picture that was drawn, indicating what the picture depicts. It is not critical to the overall meaning of the sentence, which would still be grammatically correct without it, but it enhances the reader's understanding of what kind of picture was drawn. It is not modifying a noun as an adjective would (adjective phrase), nor is it modifying a verb, adjective, or adverb as an adverb would (adverb phrase). Learn more about Appositive Phrase here: