Solution

Early developmental stages of Balanoglossus possess a spacious coelom, lined with peritoneum. It is enterocoeles in origin, as the adult condition is attained, the coelomic epithelium gives rise to the connective tissue and muscle fibers. Hence, the coelomic cavity is obliterated. The coelom as a whole is divided into three parts. These include one cavity in proboscis, (protocoel), two in the collar, (mesocoel) and two in the trunk (metacoel).
So, the correct answer is option A.