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.
