Why is A incorrect?

Thanks for your questions @Shiyi-Zhang Answer choice A is incorrect because the language is too strong. The theory of deterrence as described by the stimulus is about "belief" of another's country's retaliatory ability. Answer choice A discusses "certain knowledge," which is much stronger than belief. Certain knowledge is not required Hope this helps!

Could someone explain why C is not the best answer choice?

Hi @AllieC C is not the best answer choice because it strongly relies on an assumption of the nation's beliefs. This answer choice says that if one nation does not attack another nation, this "establishes" that they believe that they could not withstand a retaliatory attack from the other nation. The fact that one nation does not attack another does not establish that they believe this. There are many other reasons why one nation may choose not to attack another. Additionally, the question states that "fear of retaliation makes a would be aggressor nation hesitate before attacking and is 'often' sufficient to deter it altogether from attacking." As the passage uses "often" and not "always," this means that there are situations where the fear of retaliation alone is not sufficient to deter one nation from attacking this other. Hope this helpful! Let us know if you have any further questions.