Here's why:

* Clastic rocks are formed from fragments of other rocks. Chalk is not made from fragments.

* Chemical rocks are formed by the precipitation of minerals from water. While chalk does involve some chemical processes, its primary component is the skeletal remains of microscopic marine organisms called coccolithophores.

* Organic rocks are formed from the accumulation and alteration of organic material. Chalk perfectly fits this definition as it's primarily made of the fossilized remains of these tiny organisms.

Therefore, chalk is categorized as an organic sedimentary rock.