Among KO2,Na2O and BaO2 unpaired electron(s) is/are present in:
KO2 only
This essentially boils down to the differences in the magnetic nature of the normal oxide O−2, the peroxide O2−2 and the superoxide O−2 ions. Among these - the oxide and the peroxide do not contain unpaired electrons whereas the superoxide is paramagnetic (it has an unpaired electron).