Explanatory Answer:
C. The sentence in which they exist would make sense without them.
A restrictive clause is a type of subordinate clause that is essential to the meaning of the sentence, and the sentence would not make sense without it. It provides important information that is necessary to identify the noun it modifies.
Restrictive clauses do not begin with a relative pronoun (option A). Instead, they often begin with words such as "that," "who," "whom," "whose," or "which."
Restrictive clauses also do not modify verbs (option B). Instead, they modify nouns or pronouns.
Restrictive clauses are not set apart from the rest of the sentence with punctuation such as parentheses, commas, or dashes (option D). Instead, they are usually not set off by any punctuation or are separated by only a comma or two from the rest of the sentence.