The lateral surface area of a cube of side a is ______ .

The correct option is C $4a^2$
Lateral surface area of a cube is the sum of areas of 4 of its faces, excluding the areas of top and bottom faces.
The area of each face of a cube of side 'a' is $a^2$.
$⟹$ The sum of areas of 4 faces = $4a^2$
$\therefore$ Lateral surface area of a cube of side 'a' is equal to $4a^2$ .