Which increases the rate of a chemical reaction? a. a catalyst b. a higher volume c. a lower pressure d. a lower temperature

A catalyst will increase the rate of a chemical reaction by changing the reaction mechanism so that the reaction has a lower activation energy.  

a higher volume will decrease the concentrations which will intern decrease the number of collisions which will slow down the rate of the reaction since collisions between reactants are what make reactions happen.  A lower temperature causes the molecules to slow down thereby lowering the collision rate which will lower the reaction rate.  A lower pressure will lower the number of collisions which will intern result in a lower reaction rate (this can be thought of as a lower concentration since lower pressures involve less gas per unit volume)

A. A catalyst because it increases the time that it takes to happen