The correct answer is: c. chordates are more closely related to echinoderms than to arthropods
The Closest nonchordate relatives to Chordates are Echinodermata. Hemichordates (such as acorn worms) and Echinodermata form the Ambulacraria which is the sister phylum of the Chordates.
Echinoderms and chordates are both deuterostomes, but the differences are that Echinoderm possess bilateral symmetry only as larvae, they have tube feet and their bodies are supported by calcite skeletons.