The reactants are A
the substances present when the reactions begins.

B
the substances present when the reaction is complete.

C
the properties that change after the reaction.

D
the energy that is released or stored during the reaction.

The reactants are simply the substances present when a reaction begins.

Reactants are as opposed to products. The latter are substances that are present when a reaction is complete.

Reactants generally undergo chemical changes before they can form products. The chemical changes involve the breakage of old and the formation of new bonds among atoms of different elements that make up the reactants.

Reactants also need to summon an energy barrier before products can be formed. the energy barrier is known as activation energy.

More on reactants can be found here: link

A is the answer! A chemical reaction always starts with the reactants and ends with a product!

Hope this helps x :)

Explanation: