Final answer:
Pelagic organisms use buoyancy and reduced bone density to prevent sinking, examples being swim bladders in fish and lighter skeletal structures. Hence the correct answer is option a and b.
Step-by-step explanation:
The adaptations used by pelagic organisms to keep them from sinking are buoyancy (a) and reduced bone density (c). These adaptations evolved to help these organisms conserve energy and maintain their position in the water column without constant swimming. Buoyancy can be achieved through structural features such as swim bladders in fish, which can be filled with gas to adjust buoyancy. Similarly, reduced bone density decreases the overall weight of the organism, making it easier to stay afloat.
Contrary to helping with buoyancy, increased body weight (d) would actually cause an organism to sink. Counter-shading (b) is an adaptation for camouflage, not buoyancy. Examples like submarines adjusting their density through ballast tanks and helium-filled balloons demonstrating buoyant effect are analogous to biological mechanisms that control buoyancy in pelagic organisms.