Which one of the following is not a solution?

A solution is a homogeneous mixture where a solute is completely dissolved in a solvent. Alloys are classified as solid solutions because they consist of a solute dissolved in a solid solvent. Air is a gaseous solution of various gases like nitrogen and oxygen. Sugar (sucrose), however, is a pure chemical compound and a crystalline solid, not a mixture or solution by itself. While sugar dissolved in water forms a 'sugar solution' [1], the term 'sugar' alone refers to the pure substance. Notably, milk is often classified as a colloid rather than a true solution because its particles (1-1000 nm) are larger than those in a solution. However, in the context of this specific question where 'sugar' is a pure substance and the others are mixtures, sugar is the most distinct non-solution.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 9: The Amazing World of Solutes, Solvents, and Solutions > Ever heard of ... > p. 136