How can theories be replaced? ¿

If you are in a chronic state of anxiety but also suffer from moment of sudden, intense fear you are problably suffering. from

Sea stars, sea urchins, sand dollars, and brittle stars are all memeber of a group, what is the common name of the group they are all in

Ialready know the answers to these questions and they are on the attachment. but i just need to know why that answer is the correct one. plz asap! this is for a report that i need to email by 10!