IMO D

IMO E

(A) was never applied, Congress has been required to call a convention for considering possible amendments to the document when formally asked to do it ("it" is ambiguous)
(B) was never applied, there has been a requirement that Congress call a convention for consideration of possible amendments to the document when asked to do it formally ("there" is wordy & "it" is ambiguous)
(C) was never applied, whereby Congress is required to call a convention for considering possible amendments to the document when asked to do it formally (second sentence after coma should be a complete sentence & "it" is ambiguous)
(D) has never been applied, whereby Congress is required to call a convention to consider possible amendments to the document when formally asked to do so. (second sentence after coma should be a complete sentence)
(E) has never been applied, Congress is required to call a convention to consider possible amendments to the document when formally asked to do so.

Hi,
IMO E
'to do so' is the correct usage. So, A,B,C are out and we are left with D and E.
In D, 'whereby' introduces another dependent clause and the sentence is not complete because both are dependent clauses.
In E, the first clause is dependent and the second is independent. So, the sentence is complete.

Thanks smackmartine and Frankenstein.....both of you are spot ON....!!!

Ans cannot be D
whereby sounds awkward to me

i was confused between D & E. Was not clear on usage of whereby.
Thanks Frank

+1 for E. Tenses play a major role.

IMO E, do so made the difference