The substance that takes part in a chemical reaction.

The correct option is A

Reactant

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($\mathrm{MgO}$)
• $\underset{\mathrm{Magnesium}}{2\mathrm{Mg}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Magnesium}\mathrm{oxide}}{2\mathrm{MgO}\left(\mathrm{s}\right)}$

Reactant: The substance which takes part in a chemical reaction is called reactant.

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.

• In the above reaction, Magnesium oxide($\mathrm{MgO}$) is the product.

(C) Element: Elements constitute the simplest chemical substances in which all the atoms are exactly the same.

• Example includes Helium($\mathrm{He}$), Oxygen(${\mathrm{O}}_2$), etc

Therefore, the correct option is option (A).