Hello Mehmet, how are you?
1. China is now one of the largest economies in the world and it is becoming a particular trading partner of the US.
2. China is now one of the largest economies in the world and it is becoming a significant trading partner of the US.
I would guess that the question-setter thinks that "one of the largest economies" doesn't imply a degree of exclusivity in trade ("a particular trading partner"), whereas it does imply a significant share of a trading partner's trade.
Thus since (in the question-setter's mind) #1 presents two unrelated facts, while #2 presents two related facts, #2 is more likely. (#1 seems happier if you insert a comma before "and".)
He may also be thinking that "become a significant trading partner" is a more common phrase than "become a particular trading partner".
But it seems a very difficult question, to me! It should be worth double the marks.
All the best,