Which of the following reactions involve neither oxidation, non reduction ?

Considering that oxidation involves increase in oxidation number while reduction involves decrease in oxidation number, in a redox reaction, we have: (a) Cr+6O4−2⟶Cr2+6O7−2(b) Cr2⟶Cr+3Cl3(c) Na0⟶Na+(d) 2S2+2O3−2⟶S42.5O6−2From above, we find that in reaction (a), neither the oxidation number of Cr increases nor decreases from +6.