This question and best answer are based on lots of assumptions.
First, while calculating transmitting time = L/B, where L is frame length and not the IP packet.WHY here not considered?NOTE: MTU is maxed frame payload.
->You can argue that nothing is given about frame so, we may consider L here just the packet size. This can be thought behind it and is somewhat reasonable.
Second, taking bottleneck bandwidth and not considering the time elapsed by 1st packet to transmit between A-B will give the correct answer, but is this the right approach?
->Yes, many say we considered bottleneck bandwidth as packets will just accumulate at an intermediate router, so this time(x=260*8/512 ms) will be much bigger than if we include the transmitting time of 1st packet b/w A-B (x+0.8 ms). This approach would have made sense if the message is very large.
By just looking it seems small but it completely changes the answer.
For 1st case, it will be 354.65Kbps
For, 2nd case it will be 296.29Kbps.
If we consider the frame as discussed in 1st point and solve the question by taking minimum frame length then data rate will be much smaller.
This is the best I could have thought of. Don’t know still what is correct and what is not. If you find I am missing some concepts or any arguments are welcomed.