ANS:D - duralumin

Age hardening, also known as precipitation hardening or age strengthening, is a process used to increase the hardness of certain alloys, particularly those containing aluminum, copper, or magnesium. It involves heating the alloy to a high temperature, known as the solutionizing temperature, and then rapidly cooling it to retain a supersaturated solid solution. Subsequent aging at an elevated temperature allows precipitates to form within the alloy, strengthening it. Out of the options provided, duralumin is the alloy commonly associated with age hardening. Duralumin is an aluminum alloy that typically contains copper, manganese, and magnesium. When properly heat treated, duralumin undergoes age hardening, resulting in improved strength and hardness. Therefore, the correct answer is duralumin.