The main function of Clitellum is
(a) Cocoon Formation
(b) Locomotion
(c) Excretion
(d) Copulation

Hint: This starts after copulation when the ovary matures. A membrane is secreted around the clitellum by membrane secreting glands. Inside this one sperm gets fertilized with ova to form a zygote. The young worm is developed inside this structure and comes out after 2-3 weeks.

Complete Step by Step Answer:
The clitellum is part of the reproductive system. The clitellum is a thick, saddle-like ring found in the epidermis (skin) of the worm, which is usually a light-colored pigment. For the formation of cocoon for its eggs, the clitellum secretes a viscous fluid. The main function performed by clitellum is cocoon formation. In earthworms, the clitellum is only found when the worm is sexually mature.
Earthworms are hermaphrodite animals, i.e., each individual carries both male and female sex organs. As invertebrates, they lack a true skeleton, but they maintain their structure with fluid-filled coelomic chambers that function as a hydrostatic skeleton.

The sexual organs in an earthworm are located through segments 9 to 15. Pheretima has one or two pairs of testes that are contained within sacs. They have two or four pairs of seminal vesicles that produce, store, and release sperms via the male pores. Ovaries and oviducts are seen in segment 13 that release eggs via the female pores on segment 14, while the sperm is expelled from segment 15. One or more pairs of spermathecae are seen in segments 9 and 10 which are internal sacs that receive and store sperm from the other worm during copulation.
So, the correct answer is, ‘(a) Cocoon Formation.’

Note: Copulation and reproduction are two separate processes in earthworms. Though they are hermaphrodites, each earthworm obtains sperms from a different worm. So, each earthworm is the father to offspring of a different worm and mother to its own. This ensures genetic variation.