How does a catalyst work?
by increasing the temperature of a chemical reaction
by decreasing the pressure of a chemical reaction
by increasing the concentration of reactants in a reaction
by decreasing the activation energy of a reaction

A catalyst can be defined as the substance, which helps to increase the rate of a chemical reaction without undergoing any permanent chemical change itself. A catalyst increases the rate of the chemical reactions by decreasing the activation energy of the concerned reactions. The activation energy can be defined as the minimum amount of energy which is necessarily required for a a reaction to start. When the activation energy is decreased, the chemical reaction can proceed at a faster rate.

Hence, the answer is 'by decreasing the activation energy of a reaction'.

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