Substance(s) written on the left-hand side of the chemical equation are known as

The correct option is A

Reactant (s)

The explanation for the correct option:

(A) Chemical reactions: A chemical reaction is in which the bonds are broken within reactant molecules, and new bonds are formed within product molecules in order to form a new substance.

• An example of a chemical reaction is the reaction of Magnesium($\mathrm{Mg}$) with Oxygen(${\mathrm{O}}_2$) results in the formation of Magnesium oxide($\mathrm{MgO}$)
• $\underset{\mathrm{Magnesium}}{2\mathrm{Mg}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Magnesium}\mathrm{oxide}}{2\mathrm{MgO}\left(\mathrm{g}\right)}$
• The substance present on the left-hand side of the chemical reaction is a reactant and on the right-hand side is the product.
• Reactant: The substance which takes part in a chemical reaction is called reactant.
• They are present on the left-hand side of a chemical reaction.
• In above reaction, Magnesium($\mathrm{Mg}$) with Oxygen(${\mathrm{O}}_2$) are reactant.

The explanation for the incorrect option:

(B) Product:

• The substance which is formed in a chemical reaction is called the product.
• They are present on the right-hand side of a chemical reaction.
• In the above reaction, Magnesium oxide($\mathrm{MgO}$) is the product.

(C) Reaction:

• In a reaction, reactants react to form the product.
• In above example, Magnesium($\mathrm{Mg}$) with Oxygen(${\mathrm{O}}_2$) are reacting to form Magnesium oxide($\mathrm{MgO}$).

Therefore, the correct option is option (A).