The correct option is C They have spiny skin.
The organisms of phylum Echinodermata consists of spiny skinned organisms. In Greek, Echinos means hedgehog and derma means skin. The organisms belonging to this phylum have an endoskeleton made of calcium carbonate. All the other above mentioned features like true coelom, triploblastic and marine habitat are also seen in phylum Echinodermata but they are not unique to the phylum.