Correct option is D
Heat treatment is a controlled process of heating and cooling metals to modify their mechanical properties such as hardness, strength, toughness, and ductility. One of its important applications is to produce very hard and sharp cutting edges, as required in tools like surgical knives. Processes like hardening and tempering improve the cutting ability and durability of such tools.
Other options are incorrect because heat treatment does not change the melting point significantly, does not increase the weight, and cannot convert one element into another (which would require nuclear reactions).
