In Stop and Wait we can transmit only one packet in 1 RTT.
As the name suggests, after sending 1 packet, Stop & Wait protocol stops & waits until the ACK is received or a time out occurs.
So we can transmit 1 packet or 100 bits in 1 RTT.
That is transmission rate is 100 bits per RTT, or 100 bits per 2 second.
So Transmission rate = 50 bps.
Now link utilization = Current transmission rate / rate at which we could have transmit the data using the full capacity of the link (or channel)
so link utilization = 50 bps / 100,000 bps = 0.0005
We can see here Stop & Wait can be astoundingly inefficient, hence we need sliding window protocols.
Choosing appropriate windows size in sliding window protocol leads to quite higher link utilizations.