When a descriptive word, phrase, or clause is essential to the meaning of a sentence, that element is

A descriptive element that is essential to a sentence’s meaning is restrictive: it limits or identifies exactly which person or thing is meant. For example, “the book that is on the desk” uses a restrictive clause to specify which book. “Restrictive” is therefore the correct choice; “nonrestrictive” describes information that is extra, not essential.

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