Final answer:
Echinoderms have a characteristic spiny skin, which is part of their endoskeleton made up of calcium carbonate plates and spines. These spines, along with their water vascular system and tube feet, give echinoderms their unique appearance.
Step-by-step explanation:
The structure that gives echinoderms their characteristic spiny skin is the ossicle. Echinoderms, such as sea urchins and sand dollars, have an endoskeleton made of calcareous ossicles covered by the epidermis. These small bony plates are what contribute to the spiny texture of echinoderms. The spines connected to the ossicles can be moved by small muscles for defense and are also aided by tiny stalked claws called pedicellaria, which help keep the animal's surface clean. Therefore, the correct answer is b) An ossicle.