Normalizing is a heat treatment process which is used for heating the metal to a temperature slightly above the critical temperature and then cooling in air. Thus the given process is Normalizing.

Explanation:

When a steel is heated in a furnace and then cooled in air at ordinary temperature, the process is known as normalizing. This process is commonly used to improve the mechanical properties of the steel and to relieve internal stresses.

Key Points:
- Annealing: Annealing is a heat treatment process in which a metal is heated to a specific temperature and then slowly cooled, usually to make it softer and more ductile. It is typically used to reduce hardness and improve machinability.
- Hardening: Hardening is a heat treatment process in which a metal is heated to a specific temperature and then rapidly cooled, usually by quenching in water or oil. This process increases the hardness and strength of the metal.
- Tempering: Tempering is a heat treatment process in which a previously hardened metal is heated to a temperature below its critical point and then slowly cooled. This process reduces the hardness and brittleness of the metal, while improving its toughness.
- Normalizing: Normalizing is a heat treatment process in which a metal is heated to a temperature above its critical point and then cooled in air at ordinary temperature. This process is used to refine the grain structure of the steel, improve its mechanical properties, and relieve internal stresses.

Why is the correct answer 'C' (Normalizing)?
- When a steel is heated in a furnace and then cooled in air at ordinary temperature, it undergoes a process of normalizing.
- Normalizing involves heating the steel above its critical point to ensure complete transformation of the microstructure, followed by cooling in air. This process helps to refine the grain structure, improve the mechanical properties, and relieve internal stresses in the steel.
- The cooling rate in normalizing is slower than in hardening, but faster than in annealing or tempering. This intermediate cooling rate helps to achieve the desired balance of strength, hardness, and toughness in the steel.
- Normalizing is commonly used for steel components that require improved mechanical properties, such as increased strength, ductility, and toughness. It is often used in the production of structural steel, heavy machinery, and automotive components.
- Overall, the process of heating a steel in a furnace and then cooling it in air at ordinary temperature is characteristic of normalizing, making option 'C' the correct answer.