Answer:
Option B.
Explanation:
Volume of cone is

where, r is radius and h is height of the cone.
Cup A is a cone with width 3 inches and height 6 inches. It means radius is 1.5 inches.
Substitute r=1.5 and h=6 in the above formula.

Volume of cylinder is

where, r is radius and h is height of the cylinder.
Cup B is a cylinder with width 3 inches and height 5 inches.
Substitute r=1.5 and h=5 in the above formula.

Difference between their volumes is

It means cup B can hold 21.2 cubic inches more than cup A when both are completely full.
Therefore, the correct option is B.