Which of the following reaction is possible at anode?

In a galvanic cell, the anode is the electrode where oxidation (loss of electrons) happens; it is the negative electrode since electrons are left on the electrode when oxidation occurs.
As less oxidation takes place, fewer ions enter solution, and fewer electrons are left on the electrode, the cathode—the electrode where reduction (electron gain) occurs—becomes the positive electrode in a galvanic cell.

So, in the reaction $2{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}$

As the oxidation state changes from +3 to +6, $\mathrm{Cr}$ is being oxidised, making this reaction feasible at the anode.