When hardened metal is heat-treated by a process known as tempering, what metallurgical properties are achieved?

Correct Answer: Option D — decrease in brittleness

Tempering is a heat treatment process applied to ferrous alloys to achieve greater toughness. By reheating hardened steel to a temperature below the critical point, internal stresses are relieved and the crystalline structure is modified, which significantly decreases the material's inherent brittleness while maintaining a desirable level of hardness.