Identify the element forming peroxide on reaction with oxygen.

The classification of alkali metals by their oxides is well-established. Lithium forms the normal oxide $Li_2O$, sodium forms the peroxide $Na_2O_2$, and potassium, rubidium, and caesium form superoxides $MO_2$ (where $M = K, Rb, Cs$).

Among the options:
Carbon ($C$) forms $CO$ and $CO_2$, but not peroxides.
Argon ($Ar$) and helium ($He$) are noble gases and inert to oxygen.
Sodium ($Na$) reacts with oxygen according to $2Na(s) + O_2(g) \rightarrow Na_2O_2(s)$, producing sodium peroxide.