What action will slow down the rate of a reaction?

a. Raising the temperature of the reactants

b. Decreasing surface area of a solid reactant

c. Increasing the concentration of the reactants

d. Adding a catalyst to the reaction

Raising the temperature of the reactants increases the reaction between the reactants. The kinetic energy of the molecules of the reactants collides more frequently with ach other thereby increasing its reaction. Increasing the concentration of the reactants increases the reaction rate. Adding a catalyst to the reaction increases the rate of reaction of a substance. The catalyst hastens the chemical reaction. Decreasing surface area of a solid reactant slows down the reaction rate. This is because there is lesser contact of the solvent reactant to the surfaces of the solid. So the answer is letter B.

Decreasing the surface area of any reactant will slow down the reaction.

Raising the temperature can also slow down the rate of reaction is some specific cases, but that's more like an exception.

So b.