Final answer:
The exception to solids sinking in their own liquids is water; ice floats on water due to its crystalline structure being less dense than liquid water.
Step-by-step explanation:
The student's question asks about an exceptional case where a solid substance does not sink when placed in a liquid form of itself. The answer is d.water. This is because ice (solid water) floats on liquid water, despite most solids sinking in their own liquids. This anomaly occurs due to the molecular structure of ice, which forms a crystalline lattice that is less dense than liquid water, thus allowing it to float. When it comes to other substances provided in the options, such as carbon dioxide, it is known to be denser than air and will sink in an unconfined environment. Baking soda and steel are both denser than water; however, shaped steel can float due to its displacement of water and resultant buoyant force.