Collision detection condition of CSMA/CD protocol is
Transmission Time >= 2 x Propagation Time + Transmission Time of Jam signal
Here, we are not considering Jam signal. So, it’s transmission time will be considered as 0.
This collision detection condition puts a restriction on the size of frame being transmitted:
Minimum Frame size = 2 x Propagation delay x Bandwidth
Minimum Frame size = 2*(40*10^-6)*(20*10^6) = 1600 bits = 200 Bytes.