In my opinion, C.

A) wrong comparison.
B) meaning doesn't convince me.
C) Modifier ok. meaning ok.
D) Wrong modifier.
E) Aside from using, drivers are prohibited from using... It seems to me that the construction should modify the verb, but i'm not sure it is. It seems to modify the noun (drivers). Please help here to understand if this is not the choice.

I also say C is the best answer...whats the OA?

IMO D

IMO B

My answer is B.

The relative pronount 'which' is generally preceded by a comma. So, E is out.

A. Unlike the use of headset telephones, all drivers are prohibited from using portable phones while driving.
Incorrect comparison

B. Besides using headset telephones, all drivers are prohibited from using portable phones while driving.
The comparison seems to be correct. Also, 'using' is consistent

C. Unless headset telephones, all drivers are prohibited from using portable phones while driving.
Awkward

D. Other than the use of headset telephones, all drivers are prohibited from using portable phones while driving.
Incorrect comparison

E. The relative pronoun 'which' should generally be preceded by a comma. So, this option is out.

What is the OA?

A) Unlike the use of headset telephones, which allow drivers to keep their hands on the wheel, all drivers

Modifier issue. Unlike the use modifies drivers incorrectly.

B)Besides using headset telephones, which allow drivers to keep their hands on the wheel

You are trying to compare use of headsets and use of portable phones, and also trying to show a contradiction between their use while driving. This option incorrectly uses 'besides' which means 'in addition to' and is hence not used to show contradiction. Hence there is a change in meaning.

D)Other than the use of headset telephones, which allow drivers to keep their hands on the wheel, all drivers

There is a problem of incorrect comparison. You're comparing use of handsets with all drivers.

E)Aside from using headset telephones which allow drivers to keep their hands on the wheel
we need a comma before 'which' because which is introducing a non-restrictive clause.
Also you are comparing 'use of headsets' with 'drivers', incorrect comparison.

So C is fine. The meaning does not change, the right things are being compared and also C correctly uses 'unless' to show a contradiction.

OK, I agree that C is the correct answer. 'Besides' - indicates in addition to seems to be the right reason for the option to be incorrect.

OA please...

Agree with C, but shouldn't there be a comma after 'are used'

Unless headset telephones, which allow drivers to keep their hands on the wheel, are used,all drivers are prohibited from using portable phones while driving.

There is a comma after 'are used'. I dont think the comma is underlined.

OA is C. According to the explanation, all other choices make incorrect comparison of telephones to drivers. C while does not create a correct comparison gives an option where grammatical parallel isn't necessary. It creates complete clause on it's own, with it's own subject and verb.