TC not getting logic


Here for 2nd question. I am not getting why rare.
I had thought as this:-
not fiction ; indeed not ___
So it is fiction or artificial
i.e two blank go together

My second doubt is:-

Why real cannot come in blank?

The clue for blank(ii) is something that requires deciphering , so what requires it ? maybe a puzzle

I am referring to this question
“The notion that a parasite … Phenomenon is not even ____”
In which “rare” is answer

In the first of the sentence we are saying that something in not fiction or that this thing is real and now we are further emphasizing this idea by saying that this thing is actually a common (blank ) occurrence .

But because we have not before the the blank we just can’t say common , we have to say not rare (which means common)