A substance (initial concentration a) reacts according to zero-order kinetics. What will be the time for the reaction to go to completion?

The time for the reaction to go to completion is a/k.

For zero-order reaction, $[\mathrm{A}{]}_0-[\mathrm{A}{]}_t=kt$ For the reaction to go to completion $[\mathrm{A}{]}_t=0$ Hence, $t=[\mathrm{A}{]}_0/k=a/k$.

Hence, option A is correct.