When depth is added to a 2D shape, it becomes a 3D figure.
A two-dimensional shape is exactly what it sounds like; it's a shape with two dimensions, length and width.
If depth is added to this shape, it gains a third dimension.
A 3D shape is defined as a shape or figure with three dimensions. Since our current figure applies to this definition, we can assume that our figure would become a 3D figure.
Hope this helped! :)