The correct answer is D. their.
Given that the castles are plural (meaning there is more than one castle), you need to use the plural pronoun as well, and their is the only plural pronoun among these options, so you don't really have other choice. If the noun was singular (the castle), then you would use its (never his or hers because castles are not male or female).
Answer:
Explanation:
The Jan. 6 attack on the U.S. Capitol was a security failure, an intelligence failure — or both.
How could security forces in the nation's capital be so swiftly and completely overwhelmed by rioters who stated their plans openly on a range of social media sites? President Trump had even tweeted on Dec. 19: "Big protest in D.C. on January 6th. Be there, will be wild!" Washington, D.C., is known for its multitude of law enforcement agencies — a fact reflected in the agencies involved in security on Jan. 6. The Metropolitan Police Department has jurisdiction on city streets; the U.S. Park Police on the Ellipse, where Trump's rally took place; the U.S. Secret Service in the vicinity of the White House; and the U.S. Capitol Police on the Capitol complex
Paragraph 2 explains decompression