n a chemical reaction, the substances present at the beginning are called:  ProductsReactantsCatalysts

In a chemical reaction, the substances present at the beginning are called Reactants.

To elaborate further:

• Reactants are the starting materials that undergo a chemical change during the reaction. They are transformed into new substances known as products.

• Products are the substances that are formed as a result of the chemical reaction.

• Catalysts are substances that speed up a chemical reaction without being consumed in the process. They are not present at the beginning as reactants but facilitate the conversion of reactants into products.

In summary, the correct answer to your question is Reactants.