Other contributors have said "The diagram above is a potential energy curve for a reaction Which number represents the effect of a catalyst on the reaction?" is the same question as "How would a catalyst change the reaction energy diagram?" If you believe that these are not asking the same thing and should be answered differently, click here
How would a catalyst change the reaction energy diagram?
The overall enthalpy change in a common reaction would not change. Only the path to get there would change. A catalyst basically lessens the activation energy required to get the reaction to take...
when a catalyst is added to a reaction the activation energy will decrease, becase the catalyst provides an alternative route for the reactants which requires less activation energy