The United States and South Korea have a strong relationship, mainly because both countries despise North Korea, which is to South Korea's immediate north. Therefore, the answer is that "The two countries are strong trading partners."
South Korea is an ally of the US, in the sense that the two have amicable relations and that there is a vivid trade relationship between them. They are not allied against Russia as neither of them is openly in a state of conflict with Russia.
So the correct answer is: The two countries are strong trading partners.
