This can be understood by knowing the difference between Whether and If.
If:
Used for conditionsWhether :
Used for choices and in sentences where the intention is regardlesse.g.
Let me know
if you're playing - In this sentence, the author is trying to say to let him know if you're playing. If you don't tell the author then he'll assume that you're not playing.
Let me know
whether you're playing
(or not) - Here the author is asking you to tell him if you're playing or you're not playing. So, you've to tell the author regardless. The use of
(or not) is optional in English (and redundant in GMAT) because whether is giving you a choice anyway.
Regardless use of whether:
I'm playing whether you're playing or not. - Here the author is trying to communicate that he is going to play even if you play or even if you don't play. Here the use of
or not is compulsory because the author is not asking anything here but he's telling.
Now coming back to option choices:
A. Whether they will scale back their orders to pre-2003 levels or stop doing business with us altogether depends on whether the changes that their management has proposed will be fully implemented.---
Whether they will (scale back their orders to pre-2003 levels
[or not] or stop doing business with us altogether) depends on whether the changes that their management has proposed will be fully implemented. This choice is perfect!!
B. Whether they scale back their orders to pre-2003 levels or whether they discontinue their business with us altogether depends on the changes their management has proposed,
if fully implemented
or not. - Usage of
if is wrong for giving the choices
C. Their either scaling back their orders in the future to pre-2003 levels, or their outright termination of business with us, depends
on their management’s proposed changes being fully implemented or not. -- This choice is rather wordy. If I had to write this choice, I would've written it down as :
Their either scaling back their orders in the future to pre-2003 levels, or their outright termination of business with us,
depends on the changes proposed by their management.
D. Whether they will scale back their orders to pre-2003 levels or stop doing business with us altogether depends
if the changes that their management has proposed become fully implemented --
if [...] then whether [...] or
whether [...], if [...] usage is completely wrong.
E. They will either scale back their orders to pre-2003 levels, or they will stop doing business with us altogether
dependent on whether the changes their management has proposed will be fully implemented,
or not. -- No helping verb before
dependent, usage of
or not is not redundant in GMAT.
So (A) is the correct answer choice.