An alloy is a homogeneous mixture of metals or a metal and a non-metal (e.g., brass, steel), so it is a solution (solid solution). Air is a homogeneous mixture of gases (primarily nitrogen and oxygen), so it is a solution (gaseous solution). Milk contains dispersed particles of fat and protein which are not uniformly distributed at the microscopic level, making it a heterogeneous mixture (colloid). Sugar itself is a pure substance (compound), but if dissolved in water, it forms a homogeneous mixture (sugar solution). However, given the options as listed, Milk is the only one that is definitively not a solution.