Final answer:
In a chemical formula, reactants are the substances present at the start of a chemical reaction and are transformed into different substances known as products. Therefore correct option is B
Step-by-step explanation:
In a chemical formula, the parts that represent substances present at the start of a chemical reaction are called reactants. Reactants are the starting materials in a reaction, and they undergo a change to form new substances. As an example, in the combustion of methane, the reactants would be methane (CH4) and oxygen (O2), which react to form the products, carbon dioxide (CO2) and water (H2O).