A catalyst

Answer

B, C

Explanation

A catalyst provides an alternative path for the reaction occuring via a different activation state.
A poistive catalyst decreases the activation energy of a reaction, so that more number of molecules are able to cross the activation energy barrier and so the rate of the reaction is increased.
A negative catalyst increases the activation energy of a reaction so that lesser number of molecules are able to cross the activation energy barrier and so the rate of the reaction is decreased.