The correct answer is:
B. Both are open to the public.
Kronborg Castle is open to the public as a national and historical monument, and Frederiksborg Castle is a national museum that is also open for visitors. The other options are not correct: Kronborg Castle is in Elsinore, while Frederiksborg Castle is in North Zealand; only Kronborg was built in the 1500s; and only Kronborg is a UNESCO World Heritage Site, whereas Frederiksborg is not mentioned as such in the text.