Here is my 2 cents.
The key here is the scope, the stimulus give a "narrow" scope, so we can assume even narrower scope, not wider, because that will not be the question discussed here.
By "narrow" I mean the possibility or scope things can be applied to the rule in the stimulus.
like, If the stimulus says this: don't do A if at least 80% possible B happens.
This means we do A only the possibility of B is 80~100%. not 20~100 or 50~100%. So 100-80=20, it is narrow.
What does this tell us? 90% of B happens, we can do A? NO! 20% of B happens, we can do A? not sure, you can do it or not, because it is not "at least 80%". This is no rule in the stimulus we can follow. ( Out of scope? )
Also, when negating this, we got: Do A even 90% B happens, which is not allowed in the stimulus. right? So we have to assume any thing that is more than 80%, not less.
Back to the question, It tell us that "nearly always" (similar to above example of 80%), so we have to assume "unlikely" as in E, becuse it is narrower. not in C, "any chance", which is way too wide (0.00001 ~ 100%)
Hope this helps