It can't be A, because the crash didn't really cause wars.
It can't be B, because no nation just "closed" their own stock trading. In fact, this isn't really how things work, even.
It can't be D, because the United Nations would not refuse to trade with other countries. Trading with other countries is part of the international politics of every single country.
So, truly, the answer is C, Many nations relied on US investment capital that dried up after the crash.
Those were disastrous times and many people and companies ran out of money. At vary least, many ran out of investment capital, therefore places that depended on that investment felt the blow just as much; and those places were many
