* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

Let's break down the options:

* Iron (Fe) is an element, not a compound.

* Carbon dioxide (CO2) is a compound made up of one carbon atom and two oxygen atoms.

* Oxygen (O2) is an element (it exists as a diatomic molecule).

* Calcium (Ca) is an element.

Therefore, carbon dioxide is the only example of a compound in the list.