When a metal is undergoing heat treatment, what happens when the metal is heated above its critical temperature?

Correct Answer: Option C — The metal undergoes a change in crystalline structure.

When steel is heated above its critical temperature, it undergoes a phase transformation where the crystalline structure changes, typically into austenite. This structural change is the fundamental requirement for subsequent heat treatment processes like hardening or annealing, as it allows for the redistribution of carbon and alloying elements.