Answer C
Total bandwidth of input channels = 100 * 1000 * 4 bits/sec
Transmission time by all four channels in 2 bit slot is = 2/ ( 100 * 1000 *4 ) = 0.000005 mlsc
now question is in one sec how many bits you get are transferring ?
That is = 1/(0.000005) *2 bits/sec = 400Kbps