The answer is fiber.
Cellulose is mainly found in plants, it makes up the plant cells' cell walls. Cellulose is made up of fiber, this also explains why we cannot digest fiber, as inside our body, we do not have the suitable enzyme to digest the cellulose component.
On the other hand, fat and steroid is some types of lipids, and polypeptides are the chains that makes up protein, which is also digestable in our body.
So your answer should be fiber.
