If a catalyst is used in a reaction,
a. the energy of activation increases.
b. different reaction products are obtained.
c. the reaction rate increases.
d. the reaction rate does not change.

C. the reaction rate increases.

Explanation:

A catalyst is a substance which increases the rate of a reaction by taking the reaction through a different path which involves lower activation energy and thus more molecules can cross the energy barrier and convert to products.

Activation energy is the extra energy that must be supplied to reactants in order to cross the energy barrier and thus convert to products.

The catalyst itself does not take part in the chemical reaction and is regenerated as such at the end.

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there are two significant figures.