Acatalyst is: a. a solid product of a reaction. b. one of the reactants in a single-replacement reaction. c. not used up in a reaction. d. the product of a combustion reaction.

A catalyst is C. not used up in a reaction.

Explanation:

A catalyst is a substance capable of promoting or accelerating a chemical reaction without directly intervening in it. That is to say that at the end of the reaction the catalyst remains unchanged. Therefore, it is not considered as a reagent or product according to the stoichiometry of the reaction.

Catalysts that are responsible for increasing the speed of a chemical reaction are called positive catalysts and are the most common and used. But there are also  negative catalysts, also known as inhibitory catalysts. These reduce the speed of a reaction.

Catalysts can be classified into:

Finally, the correct answer is a catalyst is not used up in a reaction.

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.