The volume formula of a cylinder is :

The volume formula of a cone is :

From the problem, the heights are constant or the same.
So we can let h = 1 and compare the volumes.
a. Cylinder with r = 6 and h = 1

b. Cone with r = 6 and h = 1

c. Cone with r = 3 and h = 1

d. Cylinder with r = 3 and h = 1

Comparing the volumes from least to greatest :

or c, d, b then a.
ANSWER :
C, D, B then A