The word (is) is always used as a verb in written and spoken English.
The underlined subject of the sentence the skyscraper that was built last year is a noun phrase.When determining the type of phrase, you first need to find its center, of the most prominent word. Here, the center is skyscraper, which is a noun, which means that the whole phrase is a noun phrase.
C) I took Jason—my best friend—to the Halloween party.
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