Answer:
a. The nations are former allies that have split apart in disagreement.
Step-by-step explanation:
An asunder can be defined as a process which typically involves having two people or parties that are formally united by marriage, treaty, or league going their separate ways (disintegrating or drifting apart) as a result of war, disagreement or other unfavorable conditions.
For example, it is an asunder when Russia and the United States of America who are considered to be allies (united formally) split apart in disagreement and as such would not do things in cooperation again.
Hence, the sentence in which the relationship between nations can best be described as asunder, is when nations who are former allies split apart in disagreement.