China, korea, and japan share cultural
similarities in part due to their
(1) clashes with russian imperialists
(2) shared river systems
(3) contacts through trade
(4) unification under mongol rule

The answer is C, contacts through trade. I'll eliminate the other answer choices:
A: clashes with Russian imperialists. Japan wasn't affected by Russian imperialists- it was pretty much an independent country during that time.
B: shared river systems. Japan is an island, and it is far away from China and Korea, so shared river systems are pretty much out of the question.
D:unification under Mongol rule. The Mongol empire was only in China and Korea at various times, but never Japan.