Each frame carries 1 byte from each channel; we have 4 channels
The size of each frame, therefore, is 1x4= 4 bytes, or 32 bits.
Each channel is sending 100 bytes/s and a frame carries 1 byte from each channel, the frame rate must be 100 frames per second. The bit rate is 100 × 32 = 3200 bps.
Answer D