The correct answer is: Carbon.
Carbon is the most important alloying element in steel. It is the element that gives steel its strength and hardness. The amount of carbon in steel is measured in weight percent. The higher the carbon content, the stronger and harder the steel will be. However, too much carbon will make the steel brittle.
Iron is the main element in steel. It is the element that gives steel its ductility and toughness. The amount of iron in steel is not usually specified, as it is the majority of the material.
Manganese is a common alloying element in steel. It is added to improve the strength and toughness of steel. It also helps to prevent the formation of cracks in steel.
Sulphur is an undesirable impurity in steel. It can cause the formation of cracks in steel. It can also reduce the strength and toughness of steel.
Therefore, the correct answer is: Carbon.