Question

which element is found in all the following iron oxide air milk and glass

Answer 1

You probably mean iron (III) oxide, Fe2O3. Then we have air, a mixture of N2, O2, Ar and some odds and ends (CO2 is one of them). Milk is H2O (water) plus some very complex molecules called proteins, which are generally made of carbon, hydrogen, oxygen and nitrogen. Finally, glass is mostly silica, SiO2, with various additives, sodium being particularly common.

See the common element? it's the most common element on the surface of the Earth, and in fact one of the most common elements in the universe.