When the concentration of a reactant is decreased in a chemical reaction, the reaction rate will generally decrease. This is because a lower concentration of reactants means that there are fewer molecules available to collide with one another, which is essential for the reaction to occur.
In chemical reactions, the rate at which reactants turn into products depends on the frequency of collisions between reactant molecules. According to the collision theory, for a reaction to take place, the reactant molecules must collide with enough energy and with the proper orientation. If there are fewer molecules available due to a decrease in concentration, the number of successful collisions per unit time decreases, leading to a lower rate of reaction.
Therefore, the correct answer is a) decreasing the rate of reaction.