A substance which increases the rate of a reaction and remains unchanged is called,

The correct option is C

Catalyst

The explanation for the correct option:
(C) Catalyst:

• A catalyst is a substance that accelerates a chemical reaction by lowering the activation energy without itself being consumed by the reaction.
• iron: Iron is used as a catalyst in the preparation of ammonia.
• This reaction, also known as the Haber process, converts nitrogen and hydrogen into ammonia.
• Iron accelerates the reaction by breaking nitrogen bonds to make it more reactive.
• ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\stackrel{\mathrm{Iron}}{\to }2{\mathrm{NH}}_3\left(\mathrm{g}\right)∆\mathrm{H}=-92{\mathrm{KJmol}}^{-1}$

The explanation for the incorrect options:
(A) Reactants:

• The substance that takes part in a chemical reaction and undergoes change during a reaction is known as reactants.
• Reactants are the substances that are present on the left side of the arrow in a chemical equation.

(B) Products:

• The substances that are produced at the end of the reaction are known as products.
• Products are the substances that are present on the right side of the arrow in a chemical equation.
• Example: Hydrogen$\left(\mathrm{H}\right)$ reacts with Oxygen$\left(\mathrm{O}\right)$ to produce Water$\left({\mathrm{H}}_2\mathrm{O}\right)$.
• In this reaction, Hydrogen and Oxygen are the reactants and Water is the Product.

Hence, Option (C) is correct. i.e., A substance that increases the rate of a reaction and remains unchanged is called a catalyst.