The question asks to identify the first coelomate phylum in the animal kingdom. A coelom is a fluid-filled body cavity completely lined by mesoderm, which provides space for organs and allows for greater body flexibility. Let's examine the options: A) Echinodermata: Echinoderms (like starfish and sea urchins) are coelomates, but they belong to the deuterostome lineage, which evolved later than the first protostome* coelomates. B) Mollusca: Molluscs (like snails and clams) are coelomates, but their coelom is often reduced. They appeared evolutionarily after annelids. C) Arthropoda: Arthropods (like insects and spiders) are coelomates, but their coelom is highly reduced, and their main body cavity is a hemocoel. They also appeared after annelids. D) Annelida: Annelids (segmented worms like earthworms and leeches) are protostomes and are widely recognized as the first animal phylum to evolve a true, well-developed coelom. This coelom is typically segmented and plays a crucial role in their hydrostatic skeleton. Therefore, Annelida is considered the first coelomate phylum. The final answer is $\boxed{\text{D}}$ Last free one today — make it count tomorrow, or type /upgrade for unlimited.