Correct Answer: C
Explanation:
A careful reading of the sentence makes it apparent that the missing word is an antonym of stabilize as it has been clearly brought out that the neighbouring countries are now working to stabilize the country quite unlike what they were doing in the past. Therefore, A and B are clearly incorrect. Out of the remaining three options, subvert and destabilize are the only synonyms and they are also the antonyms of stabilize. Therefore, C and D are the correct answers.