Which one of the following statements is correct?

1. Statement A is incorrect. Echinoderms (like starfish, sea urchins) reproduce sexually with external fertilization, releasing eggs and sperm into the water. Most are oviparous (egg-laying), and while some species exhibit brooding behaviour or have larval stages that develop internally, they are not generally viviparous (giving birth to live young developed internally). 2. Statement B is correct. Roundworms (Nematodes) have a relatively simple body plan. They possess digestive and reproductive systems but lack a discrete circulatory system (heart, blood vessels) and respiratory system. Nutrient and gas transport occurs via diffusion across the body cavity. 3. Statement C is correct. Most bony fishes (Osteichthyes) possess a gas-filled organ called a swim bladder, which helps them control buoyancy and maintain depth in water usually without continuous swimming. 4. Statement D is correct. In cartilaginous fishes (Chondrichthyes) like sharks and rays, fertilisation is typically internal, with males using claspers to transfer sperm to the female. 5. Reconsidering the answer 'a': If A is the correct answer, it implies B, C, and D are incorrect. This contradicts established biological facts (B, C, D are generally considered correct). Statement A is definitively incorrect as most echinoderms are not viviparous. There seems to be a significant error in the provided answer key 'a'. Assuming the question seeks the correct statement based on biology, options B, C, and D are all strong candidates for being correct. *Analysis highlights the factual correctness of B, C, and D and the incorrectness of A, indicating an issue with the provided answer key 'a'.*