IMO E.

Core structure:

Medical researches believe something is the cause of birth defect.

who have identified a genetic abnormality in parents of children with Downs syndrome - Modifier correctly modifies researchers.

As written, the sentence says "Medical researches may be the cause of this congenital birth defect" This is incorrect.
So A and B eliminated.

C : believe this abnormality and not the age of the mother as being The part of the sentence after "and" should stand alone as clause. Here, it is not.

Between D and E:
E is correct.
Comparison between "this abnormality" and "the age of the mother" is correct.
D is incorrect.
Comparison between "this abnormality" and "other than how old the mother is" seems incorrect.

Struck between D and E. Opted E. Correct me if I am wrong

Medical researches, who have identified a genetic abnormality in parents of children with Downs syndrome , which they believe , instead of the age of the mother , may be the cause of this congenital birth defect .

(A)which they believe , instead of the age of the mother , may be
=> Incorrect use of "which"

(B)which they believe , may be more important than the age of the mother as
=> Incorrect use of "which"

(C)believe this abnormality and not the age of the mother as being
=>that is missing; error in red

(D)believe that this abnormality , other than how old the mother is , may be
=>"other than" is less idiomatic

(E)believe that this abnormality , rather than the age of the mother , may be
=> correct

This is from Kaplan. right?

Thanks guys for your response .

OA : E .Yes source is Kaplan .

@hariharakarthi nice explanation man .

As written, the sentence says "Medical researches may be the cause of this congenital birth defect" This is incorrect.
So A and B eliminated.

I missed this part completely. Thanks for that one .

@akhp77

(A)which they believe , instead of the age of the mother , may be
=> Incorrect use of "which"

(B)which they believe , may be more important than the age of the mother as
=> Incorrect use of "which"

Can you please explain why is use of WHICH incorrect in the above sentence .

Thanks .