If you get the idea right, it will seem simple to you.

You have a sentence starting with "Unlike emergency calls that .... , ". Relative pronoun "that" and the text after "that" till the comma can be ignored completely because "that...," is a modifier which can be chopped off.

It is simply "Unlike emergency calls .. , cellular calls..". Now you're down to A and B.

A is unnecessarily wordy and usage of which in that context is inappropriate. So, it's B

OMG
Reaally good effort by the GMAT test makers to make this sentence as confusing as ever!

Seriously, this question is not as tough as the attachment makes it...

For everyone's benefit...here is the question...

Unlike emergency calls that travel through regular telephone lines, where they thus automatically inform the operator of the location and phone number of the caller, cellular calls require emergency operators to determine the location of the caller.

(A) lines, where they thus automatically inform the operator of the location and phone number of the caller, cellular calls require emergency operators to determine the location of the caller

(B) lines and thus automatically inform the operator of the location and phone number of the caller, cellular calls require emergency operators to determine the location of the caller

(C) lines, thus automatically informing the operator of the location and phone number of the caller, the location of the caller on a cellular phone has to be determined by the operator

(D) lines, and thus automatically inform the operator of the location and phone number of the caller, emergency operators have to determine the location of the cellular phone caller

(E) lines, thus automatically informing the operator of the location and phone number of the caller, emergency operators receiving a cellular call have to determine the location of the caller

IMHO the Solution:

Concept tested... - Faulty Comparison

Unlike X..........., Y (Y has to immediately follow the comma)

A, C and E violate the rule

B and D follow the rule, but D compares emergency calls to emergency operators..while the actual comparison is between emergency calls and cellular calls

I safely chose B

It would be helpful if thread starters post questions that are easy on the eye. tx