Please explain!

Unfortunately...it's not true that half the radius would mean half the volume

Let's find the original radius

904.32   =  (4/3) pi  * r^3     divide both sides by   (4/3)pi

904.32

______    =  r^3

  (4/3)pi

215.89  =  r^3

Take the cube root  

6   =  r^3

Half of this   =  3

So...the volume of a ball with half of the original radius is

(4/3) pi  (3)^3   = 

36 pi  =  113.09 in^3     ...the answer that is closest is

113.04  in^3

[ The  slight difference may be due to using 3.14 for  pi  ]

Here's another way to do this

The  new volume will  be   (1/2)^3  =  1/8  of the original volume....so

904.32  * (1/8)   =  113.04  in^3