Unlike the use of headset telephones,which allow drivers to keep their

C wins. Only C correctly expresses contradiction in meaning of the sentence. Correct usage of "unless".

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I`m not expert but i will try to explain this:
Unlike the use of headset telephones,which allow drivers to keep their hands on the wheel, all drivers are prohibited from using portable phones while driving.

I go for C because are using unless, that i believed are saying that "only if I use of headset telephones".... i`m allowed to use while driving, if not use headset i will not be allowed.

So unless to me is something as only if i use headset i will be allowed to drive while talking on the phone!

E is absolute wrong as which should have comma before it.

Yes, you are right about that. E is wrong since it should have a comma.
D is wrong because of the word allows. Telephones (plural) --> allow (singular) so should be allow
C - Correct, because it signifies the meaning of the sentence. The sentence is saying: Anything that allows the driver to keep their hands on the wheel is fine, other than that (or anything else) is prohibited. Also you're comparing headphones with drivers which is wrong
B - Trap answer because the sentence is like saying : you can use headset telephones (besides) AND other things are prohibited. Be careful of this answer. It might look right, but it should also convey the meaning of the sentence!
A - wrong. Comparing a noun and a verb. The use of unlike is wrong

I got similar question on mba.com as Gmat Practice question ... can some expert throw some light what's the deifference between the option B and C ... I made C my answer which was wrong

What is wrong with E?
Is it because aside from = in addition to? => a noun must be followed because it is a preposition?
Except for is also incorrect?

KAPLAN OFFICIAL EXPLANATION:

The sentence creates a comparison between headset and all other portable telephones. While one would normally look to create a parallel structure and, starting with the fact that the nonunderlined portion begins with all drivers, start the underlined portion with a parallel reference to drivers, no answer choice gives you that option. Therefore, look for a choice that lets you off the hook and doesn't require the two parts of the sentence to be parallel. The initial sentence is flawed because it compares the use of headset telephones to all drivers, and you know by now that "comparison" constructions must compare or contrast like objects (telephones to telephones, drivers to drivers). (B), (D), and (E) repeat the same error; each creates a comparison between using headsets and drivers. (C), while it doesn't create a correct comparison, gives you an option where a grammatical parallel isn't necessary. Choice (C) creates a complete clause on its own, with its own subject and verb (Unless headset telephones...are used), so it can stand apart from the rest of the sentence. Complicated, yes, but apply what you know about comparisons to get rid of the other four choices and you'll only be left with (C).

C is the only one that actually expresses what the sentence is supposed to be saying, that drivers are prohibited from using portable phones while driving, unless they use headset telephones as they allow the drivers to keep their hands on the wheel.

Could someone please explain why option D is incorrect?

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