The underlined phrase of a sunset functions as an adjective phrase in this sentence.
It modifies the noun picture, which is why it acts as an adjective of the sentence, or rather an adjectival phrase. Appositives are usually separated from the rest of the sentence using commas. Adverbs and adverb phrases modify adjectives and verbs, and since here the center is a noun, the correct answer cannot be an adverb, but rather an adjective.
