Explanation:

- Alloy: A homogeneous mixture of metals, or a metal and a non-metal, forming a solid solution.

- Milk: A colloidal mixture of fat globules within a water-based fluid, constituting a stable solution.

- Air: A gaseous solution of various gases like nitrogen, oxygen, and small amounts of others mixed evenly.

- Sugar: A pure substance composed of only sugar molecules. It is not a mixture or solution.

Answer: Option 4, Sugar