The correct answer is option 4 - Earthworm. Clitellum is a specialized glandular region found in the body of earthworms. It is a raised and thickened band of segments located closer to the anterior end of the earthworm`s body. The clitellum plays a crucial role in the reproduction of earthworms. It secretes a sticky, mucous substance that helps in forming a protective cocoon during the process of reproduction. Inside this cocoon, fertilization and development of the earthworm embryos take place. Although the other options (leech, snail, Nereis) are all types of invertebrates, they do not possess a clitellum. Each of these organisms has their own unique reproductive structures and mechanisms.