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

Substitute the values we get

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