Whst element of art is used to describe the solidity of a form, such as that of large boulders or the sculpture House by Rachel Whiteread?

Mass.

Explanation:

There are several elements of art that we can use to describe various forms, some of which include volume, texture, shape, mass, and color.

Volume in art refers to dimension; texture to the material; shape is an obvious one; mass refers to form; and color is also an obvious choice.

If you take a look at the sculpture House by Rachel Whiteread below, you will see that it consists of large boulders, and the most notable thing about them is their solidity, or their mass given how large and massive they are.