we know that
If it is to the nearest centimeter the largest diameter it could have actually been was 20.5 cm so:
diameter=20.5 cm
radius=20.5/2-----> 10.25 cm
[volume of a sphere]=(4/3)*pi*r³------> (4/3)*pi*10.25³----> 4510.87 cm³
volume = 4511 cm³
the answer is
the largest possible volume of the ball is 4511 cm³