I'll go for C too.
My reasons for selecting C.
1.Remove the part withing the commas and the sentence should make sense.
c)The existence of a large number of electronic mail systems
, which have hundreds of thousands of subscribers, proves that while Americans may write fewer letters, another method of communication has arisen to take the place of writing letter
2.GMAT almost always wants a comma before which.
510 on my first GMAT. 610 on second GMAT.!
The struggle continues.