One of the starting substances (ingredients) that is involved in a chemical reaction

Explanation:

In a chemical reaction, the starting substances are known as reactants and the ending substances are known as products.

When we write a chemical reaction equation then substances present on the left hand side of arrow are the reactants. Whereas the substances present on the right hand side of arrow are the products.

For example

Here, both A and B are the reactants whereas AB is the product.

Thus, we can conclude that one of the starting substances (ingredients) that is involved in a chemical reaction are reactants.

the answer would be a