D. You ... buy a ticket to visit the museums in Tsaritsyno.

1) can't. You cannot visit the museums in Tsaritsyno without a ticket, so this answer is possible. 2) mustn't. You must buy a ticket to visit the museums in Tsaritsyno, so this answer is possible. 3) should. You should buy a ticket to visit the museums in Tsaritsyno, so this is the most probable answer. Correct answer: 3