Which phylum has members that secrete a hard, calcium carbonate shell?

- Arthropoda

- Nematoda

- Cnidaria

- Mollusca

i believe your answer would be Mollusca.

Explanation:

(i looked it up on britannica if you have any questions.) Mollusk, also spelled mollusc, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body.