The rate of reaction depends on various factors such as temperature, pressure and concentration.
The temperature will increase the rate of reaction in an endothermic reaction, and decrease it in an exothermic reaction.
Pressure will affect the rate of reaction for a reaction that occurs in the gas phase. Volume can also affect whether gases are present, as less volume will increase pressure.
Since we do not have data on the reaction regarding the phase of the compounds, nor on the type of reaction (Exothermic or endothermic), you cannot know how changing these conditions will affect the reaction rate.
What we do know is that as the concentration of the reactants decreases, the reaction rate decreases. We are told that the concentration of reactants is reduced by half, therefore this will be the safe option that will cause the reaction rate to decrease.
Answer: B. half the concentration of reactants