Answer: reducing the surface area
Step-by-step explanation:
a) adding a catalyst: Adding a catalyst increases the rate of reaction as catalyst lowers the activation energy and thus more reactants can cross the energy barrier. Thus increasing the rate of reaction.
b) reducing the surface area: If the reactants are present in larger size, thus having lesser surface area. Thus the rate of the reaction will be less as lesser surface will be available for reaction.
c) raising the temperature: Increasing the temperature increases the energy of the reactants and more it can interact and thus increasing the rate of the reaction.
d) increasing the amount of reactants : Rate law says that rate of a reaction is directly proportional to the concentration of the reactants. Thus increasing the amount of reactants increases the rate of reaction.