Identify the element forming peroxide on reaction with oxygen.

Step-by-step solution

The classification of alkali metals by their oxides is well-established. Lithium forms the normal oxide Li₂O , sodium forms the peroxide Na₂O₂ , and potassium, rubidium, and caesium form superoxides MO₂ (where M = K, Rb, Cs ). Among the options: Carbon ( C ) forms CO and CO₂ , but not peroxides. Argon ( Ar ) and helium ( He ) are noble gases and inert to oxygen. Sodium ( Na ) reacts with oxygen according to 2Na(s) + O₂(g) Na₂O₂(s) , producing sodium peroxide.