In this question, we are asked to identify which option is not a solution. A solution is a homogeneous mixture of two or more substances. Let`s take a look at each option to determine if it is a solution or not.

Option 1: Alloy - An alloy is a mixture of two or more metals. It is considered a solution because the metals are uniformly mixed at the atomic level.

Option 2: Milk - Milk is a mixture of water, fat, protein, lactose, and other substances. It is also considered a solution because all the components are dispersed evenly throughout.

Option 3: Air - Air is a mixture of gases, primarily nitrogen, oxygen, carbon dioxide, and trace amounts of other gases. It is considered a solution as the gases are evenly distributed.

Option 4: Sugar - Sugar, in its pure form, is a crystalline solid. It does not dissolve in water completely, but rather forms a suspension. Therefore, it is not considered a solution.

Based on the above explanation, the correct answer is Option 4 - Sugar. It is not a solution because it does not dissolve completely in water, unlike the other options.