* Mixture: A combination of two or more substances where each substance retains its own properties. Air is made up of different gases like nitrogen, oxygen, carbon dioxide, and others. These gases are not chemically bonded together.

Let's look at why the other options are incorrect:

* Compound: A substance formed when two or more elements are chemically combined in a fixed ratio. Air's components are not chemically bonded, they are just mixed together.

* Solution: A homogeneous mixture where one substance (solute) is dissolved in another (solvent). While some gases in air can dissolve in water, air itself is not a solution.

* Alloy: A mixture of metals. Air doesn't contain any metals.

* Amalgam: A specific type of alloy that contains mercury. Air doesn't contain mercury.

In conclusion, air is a mixture because it's composed of multiple substances that are not chemically combined.