Chalk formed from sediments made of skeletons of microscopic living things in the ocean must be a biogenic sedimentary rock.

Here's why:

* Biogenic means "originating from living organisms". Chalk is primarily composed of the fossilized remains of tiny marine organisms called coccolithophores.

* Sedimentary rock is formed from the accumulation and cementation of sediments. These sediments can be derived from various sources, including the remains of organisms.

So, chalk is a sedimentary rock formed from the accumulation of the skeletal remains of microscopic organisms, making it a biogenic sedimentary rock.