Which species contain unpaired electron?
BaO2
here oxidation number of Ba is +1
Ba(+) has 1 electron in its outermost shell so its unpaired
in KO2 . .oxd state of K is +1 so no unpaired electrons(noble gas config for K)
in AlO2(-) . . oxd state of Al is +3 so no unpaired electrons
in NO2(+). . oxd no of N is +5 so no unpaired electrons(noble gas config for N)