A, sodium ion has the highest valence, not reduction, but oxidation, so A is wrong;

B, sodium salt solution can't occur metal replacement reaction, so B is wrong;

C, sodium reacts with oxygen at room temperature to generate sodium oxide, and sodium peroxide is generated under heating conditions, so c is correct;

D, sodium potassium alloy has a low melting point, usually in liquid state, and can be used as a heat conductor of an atomic reactor, so D is wrong;

So choose C.