for S0=2 EX-OR Gate Delay = 20*2 = 40 ns
Z1=1 EX-OR + 1 AND + 1 OR Gate Delay = 20 + 15 + 10 = 45 ns
for next 3 FA, the carry propagation delay will be = 1 AND + 1 OR Gate Delay = 15 + 10 = 25 ns
Time taken to stabilize the output = Total Propagation Delay = 45 + 25*3 = 120 ns
Here i've also considered the time to stabilize for the final carry bit Z4