The correct answer is Quenching to form martensite.

- In steel heat treatment, the microstructure directly influences the mechanical properties such as hardness, strength, and toughness.
- Different heat treatment methods lead to different microstructures:

- Annealing produces pearlite, which is a relatively soft and ductile lamellar mixture of ferrite and cementite. It improves machinability but does not result in the hardest microstructure.
- Normalizing refines the grain size and produces fine pearlite, which is harder and stronger than coarse pearlite but still not the hardest.
- Tempering is usually performed after quenching and serves to reduce brittleness by transforming some martensite into tempered martensite, which is less hard but more ductile and tough.
- Quenching, typically involving rapid cooling from the austenitizing temperature, transforms austenite into martensite. Martensite is a supersaturated, body-centered tetragonal phase that is extremely hard and brittle. It forms due to diffusionless transformation and results in the highest hardness achievable in steel microstructures.

Therefore, quenching to form martensite produces the hardest microstructure in steel, making it essential in applications where wear resistance and strength are critical.

Reference: Metallurgy Fundamentals, Volume 2, Chapter 5: Heat Treatment of Steel, pp. 150-165