Let Tp= propagation delay
and Tt= transmission delay.
For 100% efficiency,
no of frames to be send = 1 + 2 * Tp / Tt
Number of frames sent = 1 + 4.687
= 5.687 (approx 6)
Since protocol used is Selective repeat,
Receiver window size = Sender window size.
No of distinct sequence numbers required
= 6 + 6 = 12
No of bits required to represent 12 distinct numbers = 4