Round trip delay is the delay involved in a round trip in a channel from sender to receiver and back to sender from receiver.Hence,
R.T.T = 2 * P.T. as we know propogation time is time spent in a channel from sender to receiver or vice versa.
According to the question,
R.T.T = T.T(Transmission Time)
Also we know that transmission time = data size / data rate
And we are given P.T./km = 10 μs and distance = 10km so P.T = 100 μs , so R.T.T = 200 μs
Given data size = 125 bytes = 1000 bits .Hence we have the equation,
103/x = 200/106 which gives x = 106 * 1000 / 200 = 5 * 106 bps = 5 Mbps
Hence , B) is the correct option.