The main difference between shape and form is that shapes are flat, 2-dimensional art elements, while forms are 3-dimensional art elements. This distinction means that shapes only have length and width, while forms have depth in addition to length and width.
Among the provided options, the correct statement that highlights this difference is: Shapes are flat 2-dimensional art elements; forms are 3-dimensional art elements.
The other statements either contain inaccuracies or do not accurately represent the distinctions between shape and form.