In my view, this is a case of necessary versus sufficient.
We know that Country X's not intervening means the whole region WILL DEFINITELY FALL. But, this does not imply that Country X's intervening in and of itself WILL PREVENT the fall under enemy influence. In other words, it may be NECESSARY for Country X to intervene, but not SUFFICIENT because there may be other factors that are required to prevent the fall of the region.
If Country X does not intervene militarily in Country Y, then the whole region will definitely fall under enemy influence.
It most logically follows from the statement above that, if Country X does intervene militarily in Country Y, then the whole region
(A) Will definitely fall under enemy influence X
(B) Will probably fall under enemy influence X
(C) Will probably not fall under enemy influence X
(D) Will definitely not fall under enemy influence X
(E) May or may not fall under enemy influence CORRECT
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"Do not pray for an easy life, pray for the strength to endure a difficult one." - Bruce Lee