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.

Heating steel above its critical temperature triggers an allotropic transformation, where the crystalline structure shifts from ferrite to austenite. This phase change is fundamental to heat treatment, allowing for the redistribution of carbon and alloying elements, which dictates the final mechanical properties of the metal upon subsequent cooling.