The prompt is essentially asking for the maximum number of 5 x 5 squares that can be cut from a larger 15 by 15 square. Since each 'row' and each 'column' of the larger square can be sub-divided into 3 'pieces' each, we have (3)(3) = 9 total smaller squares (at maximum).
What is the maximum number of pieces of birthday cake of size 5” by 5” that can be cut from a cake 15” by 15”?
  * 5
  * 10
  * 16
  * 20
  * 9