Average speed is defined as the ratio of the distance covered to the total time taken .

Average speed $s_{av}=\frac{Distancecovered}{timetaken}$

For step CD 

Initial position is $4$, final position is $0$, hence distance covered $=4m$

Total time is $6-4=2sec$

So average speed is $s=\frac{4}{2}=2m/s$

So option $B$ is correct.