Solution:
we are given that
Carla's new boots came in the shape of a rectangular prism. She made a layer of unit cubes in the bottom of the box.
The box was 12 cubes long and 9 cubes wide. It mean that length of the box is 12 and width of the cube is 9.
She used 7 layers of cubes to fill the box it mean the height of the cube is 7.
As we know the volume of a rectangular prism is given using the formula
[tex] Volume=Length*width*Height [/tex]
Substitute the values we get
[tex] Volume=12*9*7 [/tex]
Hence the correct option is
C. Multiply to find the area of the base and then multiply by the number of layers of cubes: (12 x 9) x 7