The correct option is
A Urochordates
The first sub-phylum under Phylum Chordata is Urochordata, which is also called "tunicata". The name refers to the unique outer covering or "tunic" of these animals, formed from proteins and carbohydrates. The tunic acts as an exoskeleton. In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff. Examples include Ascidia, Salpa and Doliolum.