Almost certainly the ones on the left side of the arrow (or on the bottom of the equilibrium constant expression). However, since you neglected to supply "this chemical reaction", we can't give you a better answer than that.
The substances you have at the beginning of a chemical reaction are the reactants or the reagents.
Reactants are the starting materials of a reaction. Reactants combine to form the products.
The reactants
Reactants are added into the equation to form the chemical reaction. Reactants are substances that are changed into products. Without these reactants, there would be no formula, resulting in no product.
The reactants in the reaction are A and B.
Reactants
Reactants are the substances present at the beginning of a chemical reaction. They are consumed during the reaction to yield products. Reactants typically undergo a transformation in their chemical structure or properties during the reaction.
The reactants exist before a reaction begins, and also the products are the result of the reaction of the reactants.
The starting materials in a chemical reaction are called reactants.
The reactants are on the left side of the equation, and the products are on the right side of the equation. The reactants are used up in a chemical reaction, and the products are the substances made by the reaction.
A and B
In homogenous reaction all the components/reactants taking part in the reaction form one single phase, with the reaction occurring in the bulk of this system. In heterogeneous reaction all the components/reactants taking part in the reaction are in different phases, with the reaction occurring between the delimiting surface of the components/reactants.