The correct answer is 8 billion USD surplus.
Since South Korea has a 17 billion USD of exports and imports that a worth 9 billion USD, we take the 17 billion USD minus the 9 billion USD and get 8 billion USD. Since the value is positive (above 0) than it means that it is a surplus, if it was the other way around and the value was negative (below O) than it was going to be a deficit.