The correct option is B mesoderm
Ectoderm, endoderm and mesoderm are germinal layers present in triploblastic organisms. Each layer is responsible for the formation of the different parts of the organism.
The notochord is a flexible rod-like structure derived from mesoderm. It is formed on the dorsal side during embryonic development in some animals. Those animals having notochord are called chordates and animals which do not have notochord are called non-chordates.