ANS:A - nickel

Of the alloying elements listed, nickel is the one that increases hardness without sacrificing ductility. Nickel is often added to steel to improve its toughness and ductility while maintaining or even enhancing its strength. It can also improve corrosion resistance. On the other hand, vanadium, molybdenum, and chromium are alloying elements that are known to increase hardness but often at the expense of ductility. They are commonly used to increase the strength and wear resistance of steel, but they may reduce its ductility compared to plain carbon steel or steel with less alloying elements.