Sentence Correction

Declining revenues resulting from a decrease in business travel, a source of income without which most commercial airlines could not survive, are going to force many commercial airlines to increase prices and decrease services in the coming months.
(A) a source of income without which most commercial airlines could not survive, are
(B) a source of income without which most commercial airlines could not survive, is
(C) and most commercial airlines use it as a source of income to survive with, are
(D) which is a source of income which is needed by most commercial airlines who could not survive without it, are
(E) which most commercial airlines use as a source of income without which they are unable to survive, is

Answer is A

I did not understand Kaplan's explanation.
Kaplan's explanation for D:
(D) creates an error by altering that clause so that it no longer acts as a modifier

Which cannot be used to modify?

Please explain. Thank you so much!

Hello Rich,

I typed out that sentence from Kaplan 800 book. It was referring to previous problem to explain error. I deleted Scribner's part to avoid confusion.
Do you think you can help explain why D is wrong?

Thanks!

there are 2 problems with D in fact 3
D says : ,which is a source of income which is needed by most commercial airlines who could not survive without it, are

you simply cannot use "who" for "airline"
you cannot use "which" (the bold one) in a sentence like this
and thirdly A is much better modifier ---its "noun + noun modifier" where "noun" refers to a "noun" before comma