Each year companies in United States could save as much as $58 billion annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided offices with cleaner air.

(A) Same
(B) annually if they prevented employee illness and gain as much as $200 billion through worker performance improved by simply providing
(C) annually in employee illness prevention and gain as much as $200 billion through worker performance improved by simply providing
(D) in employee illness prevention and gain as much as $200 billion through improving performance of workers if they simply provided
(E) by preventing illness among employees and gain as much as $200 billion through improved worker performance if they simply provided

The correct answer is (E) I chose it on the following grounds: -

annually is redundant because the original sentence is started with 'Each year' so (A) (B) & (C) are out. Between (D) & (E) I picked (E) because it is more concise.

Can instructors/someone - shed some more light on the grammatical aspects to choose (E) over other choices, if any.

Thanks

Actually, I think you lucked out a bit when you used concision as a deciding factor. By my count, (D) and (E) have the same number of words, and in fact the words themselves are longer in (E)--neither a bad thing nor a good thing on the GMAT.

Here are the only words that differentiate (D) and (E):
(D) in employee illness prevention .....improving performance of workers...
(E) by preventing illness among employees .... improved worker performance...

I think this choice boils down to idiom, although parallelism could be useful:

(D) reads "companies could save $58B in X...and gain $200B through Y" The phrasing "save $ in X" is not ideal...the only correct usage of this I can think of is "save $ in banks" (i.e. to specify location of saved money). That is not the purpose here.

(E) reads "companies could save $58B by preventing X...and gain $200B through Y" where the correct idiom "save $ by doing X" (i.e. to specify how money is saved) is used. Also, X and Y are parallel nouns (illness and performance) correctly completing the idioms "by preventing X (one thing) ...and through Y (some other thing)"

Thanks for helping all of us.

IN Ans D and E , how to know if the pronoun at the end (they) is referring to employess not to companies.

They refers to companies, not employees. Structure of the sentence: Companies could do X if they (the same companies) did Y. All of the stuff in X and Y is just detail, not part of the core.