Hardness of metals can be decreased by: (a) adding interstitial alloying elements (b) making glassy metals through extremely high cooling rates (c) annealing using slow cooling from high temperatures (d) cold-working or deforming at low temperatures

Option c is correct

Explanation:

The interstitial alloy elements occupy the free spaces in the metal structure and improve the symmetry of the crystalline network, for this reason this type of elements increases the hardness of the metal.

Vitreous metals are called that because they are formed by high cooling rates and form amorphous structures such as glass, such structures contain packaging of atoms of different sizes, which decreases the free spaces in the metal structure and this increases their hardness.

In option d, the work at low temperatures causes the vibrations of the atoms to decrease and compact, this makes many metals brittle but in general increases their hardness.

Option c is correct because in annealing the metal is heated to the austenization temperature, that is, the temperature at which a structural change occurs, which destroys its crystalline network and then slowly cools to maintain this structure change and by causing this disorder, the spaces between the atoms increase and the hardness of the metal decreases.