In which situation will the reaction rate increase

A) when a catalyst is added  

Explanation:

The missing options are:

A) when a catalyst is added  

B) when the temperature drops  

C) when the pressure is lowered  

D) when the molecules are large and bulky

A catalyst increases the rate of a chemical reaction without undergoing a permanent chemical change.  They provide an alternative pathway to the reaction which has a lower activation energy than the non-catalyzed mechanism, this increase the reaction rate.