Final answer:
The underlined phrase in the sentence is a prepositional phrase, which provides additional information about location, time, or manner.
Step-by-step explanation:
The underlined phrase in the sentence 'According to the letter, the wedding is June 25' is prepositional phrase. Prepositional phrases typically begin with a preposition (like 'according to') and end with a noun or pronoun. They function as adjectives or adverbs in a sentence, providing additional information about location, time, or manner.
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