All of the following elements are alloying elements in both high-strength and mild steel EXCEPT:

In the context of alloying elements in steels, it is important to recognize the roles that different elements play. Carbon, silicon, and copper are commonly found in both high-strength and mild steel due to their properties that enhance strength, ductility, and corrosion resistance.

Carbon is a primary alloying element that increases hardness and tensile strength in steel. Silicon can improve the strength and elasticity, while copper is often added to improve corrosion resistance, particularly in the presence of atmospheric conditions.

Molybdenum, however, is typically used as an alloying element in specialized high-strength steels but is not commonly found in mild steels. Mild steel generally contains lower alloying content and focuses more on iron and a small amount of carbon. While molybdenum can enhance properties like strength and toughness, particularly at elevated temperatures, it does not serve as a traditional alloying element across both types of steel. As a result, it stands out as being less common in mild steel as compared to the other elements mentioned.