Which option incorrectly matches an example with its broad category?

Correct answer: Camphor in nitrogen — liquid solution

Camphor is the solid solute, but nitrogen is the gaseous solvent. The broad category is determined by the solvent, so camphor in nitrogen is a solid-in-gas or gaseous solution, not a liquid solution. Air is gas in gas, brass is solid in solid, and salt water is solid in liquid. Thus option D is the incorrect match.