Answer:
Correct answer is (D) destroy the Concert of Europe and leave Russia without allies .
Explanation:
Option A is not correct as this two principalities were unified just a couple of years later into Romania.
Option B is not correct as this war didn't affected Estonia.
C is not correct as quite on the contrary this was one of the crisis that undermined the Concert of France.
D is correct as European forces, especially Austria, Britain and France were afraid that Russian influence and conquering of Constantinople would totally change the balance of powers.