Sentence Correction

Research during the past several decades on the nature of language and the processes that produce and make it understandable has revealed great complexity instead of underlying simplicity.
(A) that produce and make it understandable has revealed great complexity instead of underlying simplicity
(B) of producing and understanding it have revealed not underlying simplicity but great complexity
(C) by which it is produced and understood has revealed not underlying simplicity but great complexity
(D) by which it is produced and understood have revealed great complexity rather than underlying simplicity
(E) by which one produces and understands it have revealed great complexity instead of underlying simplicity

OA after few explanations[/u]

Isn't A correct?
I don't think there is any pronoun/subject verb agreement error in the sentence.
Can't be a tense error either...beats me!

IMO A. to make it understand u have to break the sentance... processes that....research on X & Y has............... as research is singular...so it should be A.

Yes the answer should be A.

OA is A

B, D, E - subject verb error, Research is singular so has should be used
C - 'by which' awakward moreover it is produced is passive

I call bull on the OA, I think its C.

the "it" is more ambiguous in A than in C. Also, processes "by which" kinda makes sense...e.g. The scientiest used processes by which he discovered the particle.

I will go with A.

It should be 'has' because of the singularity of word 'research'.
Out of the remaining 2, C does'nt suit.

I feel Ans should be D.

Research {Modifier} on the nature of language and {Additive} the processes , As additive is present, subject becomes plural and hence answer will follow... "have".

SC gurus, please correct If I'm wrong.

where is the ref of 'it' in the sentence. So, why C. IMO A.

research have revealed is wrong so B, D and E are out.

Language can not be understood by the procee rather processes make it understandable so I think A is the answer.

@vivek

i could pose the same question regarding "it" for A too, right?