(B.) False. Rwnd may change during connection. Although Reciever Buffer remain fixed.
(C.) True. Since the number of unacknowledged bytes that A sends cannot exceed the size of the receive window. So there will be no way it can exceed reciever buffer, since rwnd <= rcvr buffer.
(D.)False. It is not necessary. Let say sender sends following segment with seq. number 10-19 it gets received , 20-29 gets lost ,30-39 get rcvd, now sender can retransmit segment (20-29) instead of (40-49).
(E) True. Header contains field window size(advertised window) of 16 bit
(G.) False. it may be the case that the segment sent before the segment with sequence number (38-41) got lost. then the acknowledgement number will not be 42.