Manhattan CAT-difference between 'whether' &'whether or not'

[Aimgmat]

Hi,

Can someone please explain when should we use 'whether' and when should we use 'whether or not'?
In the below Manhattan CAT ques, what is the difference between answer choice a & b? Is the difference is just based on 'Concision' or there is something else?

Thanks!!

Ques-
To this day, researchers and theorists debate whether bubonic plague caused The Black Death, a pandemic that swept the world in the middle of the fourteenth century.

a) whether
b) whether or not
c) about whether
d) as to whether
e) if

[RonPurewal]

If "whether" works, then "whether or not" is redundant.

(Redundancy, unlike "lack of concision", is actually an error.)

[WWWL471]

Please tell me why not C. Because I remember DEBATE ABOUT, I just pick C. Thank you!

[Sage Pearce-Higgins]

Where do you remember 'debate about' from? That's not a correct idiom in English; the correct idiom is to 'debate something' (i.e. no preposition needed).

https://en.oxforddictionaries.com/definition/debate