16 is the answer
Q doesn't say that only after 3rd packet B will send theacknowledgement for 1st packet from A
So we can say
"All packets are ready and immediately available for transmission"
packet1 from A - Tranmit1
acknowledgement (ack) for packet1(pckt) from B
packet2 from A - Transmit2
ack for pckt2 from B
packet3 from A - Transmit3
ack for pckt3 from B
packet4 from A - Transmit4
ack for pckt4 from B
packet5 from A - Transmit 5 (packet5 lost, so no ack from B for pckt5)
packet6 from A - Transmit 1 (go back n, so no ack for pckt6 also)
packet7 from A - Transmit 2 (go back n, so no ack for pckt7 also)
Timeout for packet5
packet5 from A - Transmit 3
ack for pckt5 from B
packet6 from A - Transmit 4
ack for pckt6 from B
packet7 from A - Transmit 5 (packet7 lost, so no ack for pckt7)
packet8 from A - Transmit 1 (go back n, so no ack for pckt8 also)
packet9 from A - Transmit 2 (go back n, so no ack for pckt9 also)
Timeout for packet7
packet7 from A - Transmit 3
ack for pckt7 from B
packet8 from A - Transmit 4
ack for pckt8 from B
packet9 from A - Transmit 5 (packet9 lost, so no ack for pckt9)
Timeout for packet9
packet9 from A - Transmit 1
ack for pckt9 from B
B now receives all 9 packets.
In total 16 transmits from station A