Final answer:
The correct answer to the student's question is 'pack carburizing,' which is a process where carbon is added to the surface of steel to harden it.
Step-by-step explanation:
The process where carbon is added to the surface of steel to harden it is known as pack carburizing. This process involves packing the steel in an environment enriched with carbon and heating it to a temperature where the carbon is absorbed into the steel's surface. This forms a harder carbon-rich outer layer which increases the material's wear resistance and fatigue life. Decarburization, precipitation hardening, and quenching are different processes used for heat treating metals to alter their physical properties, but they do not involve adding carbon to the surface of steel.