Many scientists believe that abnormalities in brain chemistry

The sentence does not look worded properly. The same pronoun "they" seems to refer to "scientists" and to "abnormalities". What is the source of the question?

Agree with you but that problem is in all ACs and the AC with least problem is A.
So A is fine.

(A) cause clinical depression, but they have not established what these abnormalities are or
(B) cause clinical depression, but they have not established either what these abnormalities are nor
(C) are the cause of clinical depression, but they have neither not established what these abnormalities are or
(D) are the cause of clinical depression, but they have not established either what these abnormalities are nor
(E) causes clinical depression, but they have not established what these abnormalities are nor

in this question we have 2 problems

1) subject -verb agreement
2) idiom error

correct idioms are
a) either x or y
b) neither x nor y

choice b , c and d are out due to this idiom error
choice e is out since abnormailities doesn't agree with singluar verb causes

so we are left with only where subject and verb agrees and we don't have any incorrect idioms also

so my choice is A

As most people said, there's the idiom issue in most of the answer.

Either has to pair with or
Neither has to pair with nor

in (E), there's a subject/verb issue.

Here's my take on the "they" part.

Whenever I see a pronoun, I always ask myself what that pronoun is referring to. Who is "they" in this case? Is it abnormalities or scientists? Or something else? By not being able to answer myself, I cross (A) off in the list. However, once I realize that "they" persists in every single answer, I un-crossed (A) off knowing that this pronoun cannot be the make and break of this question. I completely ignored the "they" part and focused on the idiom error.

What is the source of this question?