The 19th-century proponents of the school of thought known as mechanism held that life process are not the products of some mysterious life force, but are the same chemical and physical processes that operate in inorganic systems, which is still a subject of debate between biologists today.
(A) systems, which is still a subject of debate between
(B) systems, which are still debated between
(C) systems still debated among
(D) systems, a theory still debated by
(E) systems, a theory still debated between

OA is D but I picked E. Is not 'debated between' idiomatically correct?

if this is an official problem, then you have evidence that GMAT RULES prefer 'debated by' to 'debated between'.
my personal experience would concur with striking 'debated between' as wrong, too, but GMAT RULES are the only experience that matters.

btw, 'between' itself is also an issue. since there are more than 2 biologists, 'between' would be wrong anyway in any context (the correct form being "among").

Looking at this post by Stacey, http://www.manhattangmat.com/forums/post14733.html, it seems that between is used for more than two entities. In that post OA is C.

i stand by my suspicions about the problem in that thread; it just doesn't have the vibe of an official problem, at all.

it's ok to use "between" in the context of more than 2 entities, as long as whatever you're talking about involves only 2 entities at a time. so if "relations between nations" refers exclusively to "relations" involving no more than 2 nations at a time, then you're fine.

analogy:
fights between members of the 12-and-under division of Golden Gloves rarely last longer than ten minutes.
while there are almost certainly more than 2 members of the division, this sentence still makes sense, because boxing matches are fought by only 2 fighters at a time.