The correct answer is: A. Manganese
Manganese is a common alloying element in steel, and it is used to improve the strength, toughness, and hardenability of steel. It also helps to improve the machinability of steel and to reduce the effects of sulfur.
Chromium is also a common alloying element in steel, and it is used to improve the corrosion resistance and hardenability of steel. It also helps to improve the wear resistance of steel.
Nickel is a less common alloying element in steel, and it is used to improve the toughness, corrosion resistance, and hardenability of steel. It also helps to improve the machinability of steel.
Vanadium is a less common alloying element in steel, and it is used to improve the strength, toughness, and hardenability of steel. It also helps to improve the wear resistance of steel.
In conclusion, the yield strength and tensile strength of low carbon steel may be improved by the addition of manganese.