Answer: A. adding a catalyst
Step-by-step explanation: The given reaction is -
8Zn(s) + S8(s) → 8ZnS(s)
Catalyst is the substance which is used to increase the rate of reaction. Thus adding the catalyst will give the greatest increase the rate of reaction by following the path which would have less activation energy.
Increasing the temperature will do increase the rate of reaction as kinetic energy of the reactants increasing thereby increasing the number of collisions.
Increasing the surface area and increasing the concentration will also increase the rate of reaction but the greatest increase will be shown by the addition of the catalyst.