What is the downstream throughput if two 256-QAM channels, each with a throughput of 38 Mbps, are bonded?

When two channels are bonded, their throughputs are combined to create a higher overall throughput. In this case, each of the 256-QAM channels has a throughput of 38 Mbps. When these two channels are bonded together, you simply add their individual throughputs together.

So, the calculation would be:

38 Mbps + 38 Mbps = 76 Mbps.

This indicates that bonding the two channels results in a total downstream throughput of 76 Mbps, which is the correct answer. The bonding process essentially allows for the combined transmission capacity of both channels to be utilized, effectively doubling the throughput.