Oxygen is a substance that is NOT a solution compared to beer, humid air, and steel, as it is a pure element and does not involve a solvent or a solute in its pure form.
The substance in the given options that is NOT a solution is oxygen. A solution is a homogeneous mixture of two or more substances, typically consisting of a solvent and one or more solutes homogeneously dispersed. Beer is an alcoholic beverage that contains multiple ingredients, such as water, ethanol, and various dissolved substances, which makes it a solution.
Humid air is a gaseous solution consisting of nitrogen, oxygen, and water vapor, among other gases. Steel is a solid solution, an alloy made by dissolving carbon and other elements in iron. However, oxygen by itself is a pure element, not a solution. It is composed of O₂ molecules and does not involve a solvent or a solute unless it is air where it acts as one component of a solution.