Which sentence contains an underlined restrictive clause?

Which sentence contains an underlined restrictive clause?

A.The books that are stacked neatly on the top shelf belong to my great-grandfather.

B.The girl who broke the downstairs window with a baseball is knocking on the front door.

C.The store allowed the items that were purchased less than thirty days ago to be returned.

D.The restaurant, which had its grand opening two months ago, is always crowded.

Answer: B.The girl who broke the downstairs window with a baseball is knocking on the front door.

Including the underlined restrictive clause, the following sentence can be identified: A relative clause is also called essential, because it contains informative material which specifies the noun it qualifies. Here, “who broke the downstairs window with a baseball” is another relative question to know which girl is knocking at the door. In this case, without the inclusion of this clause, the reader is unable to determine which girl is being pointed out. This is not set off by commas because the element is a part of the meaning of the sentence and adds a limitation on the rest of the phrase. Comparably, option A and C have “that” clauses that are restrictive, but these are not underlined in the question. Looking at Option D it is seen that there is a non-essential clause that uses commas to set it off hence being additional.