The correct answer is: A. Eight.
A 200 kHz channel in GSM can support up to eight users or voice channels. This is because each user is assigned a time slot within the 200 kHz channel, and there are eight time slots per frame. A frame is a fixed period of time in GSM, and it is equal to 120 milliseconds.
The other options are incorrect because they do not represent the number of users or voice channels that can be supported by a 200 kHz channel in GSM. Option B, three, is incorrect because there are eight time slots per frame, and each user is assigned a time slot. Option C, sixty four, is incorrect because there are only eight time slots per frame. Option D, twelve, is incorrect because there are only eight time slots per frame.
I hope this helps!