Option (C) is the correct answer.
In the chemical reaction, reactants are the starting elements or compounds which will chemically react and will form the products. Reactants are only present on the left hand side of the reaction and products are present on the right hand side of the reaction.
Reactants present in the reaction is completely used up in the reaction to form products implies no traces of reactants will be present in the product side.
For example:
Here, hydrogen and oxygen are the reactants whereas water is the product (new substance). Hydrogen reacts with oxygen and form water i.e. hydrogen and oxygen is completely used up in the reaction to form new substance i.e. water.
Hence, substances that are used up in a reaction is known as reactants of the reaction.