(a) Alloys are homogeneous mixtures of metals, because
(i) it shows the properties of its constituents, and
(ii) it has variable composition., e.g., brass is considered a mixture, because it shows the properties of its constituents, copper and zinc, and it has a variable composition (amount of Zn in brass can vary from 20 to 35 percent).
(b) A solution is generally a liquid, not always. It may involve solids and gases also, e.g.. alloys are a solution of solid in solid. Air is a solution of gases in gases.
(c) Colloidal solutions are heterogeneous in nature, though they appear to be homogeneous.