The correct option is C CrO2−4→Cr2O2−7
In (a), oxidation state of V changed from +4 to +3. Thus, it is reduction.
In (b), oxidation state of Na changed from 0 to +1. Thus it is oxidation.
In (d), Osidation state of Zn changed from +2 to 0. Thus it is reduction.
Cr oxidation state is not changed. Hence, neither oxidation nor reduction.