In grammar, nouns, adjectives, verbs, adverbs, prepositions, etc, are the parts of speech. The underlined subject, "the skyscraper," is a noun phrase. Thus, option b is correct.
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A noun phrase is a group of words that contains a noun as the head and the most prominent words that act as the center. Here, the whole sentence revolves around skyscrapers and hence it being a noun and defining the name of an object makes the sentence to be a noun phrase.
Therefore, the subject is a noun phrase.
Learn more about noun phrases here:
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