The correct answer is b) Sea urchins.
Sea urchins belong to the phylum Echinodermata, which is distinct from Arthropoda. Arthropoda includes animals that have exoskeletons, segmented bodies, and jointed appendages, such as spiders and millipedes. Spiders are arachnids, a class within the phylum Arthropoda, and millipedes are myriapods, also within Arthropoda. In contrast, sea urchins, which are echinoderms, have a completely different body structure and organization, characterized by a hard, spiny exterior and a unique water vascular system.