A catalyst :

Options

A. Increases the average kinetic energy of the reacting molecules.
B. Decreases the activation energy
C. Alters the reaction mechanism
D. Increases the frequency of the colliisons of the reacting species

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.