For stop and wait protocol the efficiency is 1/1+2a where a =Propagation Time/ Transmission Time
The utilization of the link i.e. the bandwidth is the throughput i.e. efficiency *the given bandwidth.
So for this question :
Tt=Length of the packet in bits/bandwidth=2*8*1000/2.5*10^6=6.4ms
Tp=20ms
efficiency=1/1+2*20/6.4
and link utilization=efficiency *bandwidth