A dancer demonstrating dance steps along a straight line. The displacement time graph is given.

Find the average speed for the dance step depicted by CD.

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.