Now back to the question

Now, it is a short sentence and it does not have so many clues as an anchor point. However, dissecting it we can come to an answer

The notion that a parasite can alter the behavior of a host organism is not mere fiction;

This sentence tells us that this phenomenon is NOT a notion we see at the cinema in a movie. Think about the icon sci-fi "The thing " with Kurt Russel. So, from this, we could have a sort of prediction of the blank: maybe what is happening with a parasite is true. It is concrete. And IF it is real and so far from reality, even though the same phenomenon is not quite common like the sun shines in the morning, the same COULD happen. Perhaps with a very low probability. Nonetheless, it could happen somewhere in this world.


indeed, the phenomenon is not even (i) _______.


According to this second part of the sentence, the phenomenon itself is NOT

observable

Saying that the phenomenon is NOT even observable is wrong. We do know from the first sentence that the phenomenon IS observable. Somewhere, somehow. NOT even observable is a contraddiction. It is.

real

We do know that is real. Otherwise about what are we talking about if it was not real ?? it is real. It is true.

comprehended

Out of scope. At the moment is not our concern if we do have a comprehension of it or less. We care about if it is true or less. BUT we do know that is true and tangible.

rare

Correct. The phenomenon is not only a thing for a movie's script. It is real. Even though it is rare and NOT common.

imaginable

It is. It is imaginable because it is real and true. It is NOT only a hypothesis that we see in a sci-fi movie. Could happen somewhere, somehow. Before or after.


Hope this helps.
Regards