Catalyst are that increase the rate of a chemical reaction.

substances

Explanation:

I would simply put it as catalysts are substances that increase the rate of a chemical reaction.

Keep in mind there are numerous synonyms for substances such as things etc. which could defintely be applicable as an answer therefore, the answers might vary.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.