The correct answer is: B. Carbon
Carbon is a common impurity in steel, and it can cause brittleness when present in excess. This is because carbon atoms can form small clusters, or carbides, which can act as stress concentrators. When a material is subjected to stress, these carbides can cause the material to fail prematurely.
Sulphur, phosphorus, and silicon are also common impurities in steel, but they do not typically cause brittleness in the same way that carbon does. Sulphur can form inclusions, which can also act as stress concentrators, but they are not as common as carbides. Phosphorus can make steel more brittle, but it does so by increasing the hardness of the steel. Silicon can also make steel more brittle, but it does so by reducing the ductility of the steel.
In conclusion, the correct answer is: B. Carbon. Carbon is a common impurity in steel that can cause brittleness when present in excess. This is because carbon atoms can form small clusters, or carbides, which can act as stress concentrators. When a material is subjected to stress, these carbides can cause the material to fail prematurely.