Final answer:
Option (a), The conversion of a closed cavity into an open pouch is termed Evagination. This is different from diverticulosis and other similar terms.
Step-by-step explanation:
The term used to describe the conversion of a closed cavity into an open pouch is a) Evagination. This term is not to be confused with diverticulosis, which is a condition where pouches called diverticula form in the wall of the large intestine, nor with invagination, which is the folding of a part of the body or tissue to the inside, nor with perforation, which generally refers to a hole forming through an organ or tissue.
In the context of the alimentary canal, which consists of organs throughout which various processes such as propulsion, segmentation, and absorption occur, evagination does not typically pertain to the normal functions but could be associated with pathological changes. It's important to remember that an alimentary canal has two openings: a mouth and an anus, as opposed to a gastrovascular cavity which has a single opening serving as both mouth and anus.