If each small cube in the given figure has a volume of one cubic unit, the volume of the cuboid is cubic units more than that of the cube.

The correct option is C $4$
For this we will find out the volumes of the cube and the cuboid.

Given each small cube has a volume of $1$ cubic units.

$\therefore$ Volume of the cuboid will be the number of the cubes inside the prism.

Here, there are a total of $12$ cubes in the structure.
$\therefore$ Its volume will be $12$ cubic units.

Similarly, for the cube, we have a total of $8$ small cubes inside its structure.

Thus, its volume will be $8$ cubic units.

Therefore, the volume of the cuboid is $12-8=4$ cubic units more than that of the cube.