The correct answer is D) decrease in brittleness. When hardened metal is heat-treated through the process of tempering, the primary metallurgical property achieved is a decrease in brittleness. Tempering involves heating the hardened metal to a specific temperature, which allows some of the internal stresses to be relieved, resulting in a more ductile and less brittle material. This improvement in ductility is the key benefit of the tempering process. The other answer choices are incorrect because: A) tempering decreases brittleness, not increases it; B) tempering does not directly affect corrosion resistance; and C) tempering does not increase the hardening of the metal, but rather reduces the excessive hardness achieved through the initial hardening process.