ANS:A - carbon

The 'transition temperature' for ductile to brittle behavior of steel increases with an increase in the carbon content in steel. The transition temperature refers to the temperature at which a material undergoes a transition from ductile behavior (characterized by plastic deformation and energy absorption before fracture) to brittle behavior (characterized by sudden and catastrophic fracture with little or no plastic deformation). In steels, the transition temperature is influenced by several factors, including the composition of the steel. Higher levels of carbon in steel tend to increase the transition temperature. This is because higher carbon content can lead to the formation of carbides, which act as nucleation sites for brittle fracture and reduce the ductility of the steel. While manganese content can also influence the mechanical properties of steel, it typically has a lesser effect on the transition temperature compared to carbon. Therefore, the transition temperature for ductile to brittle behavior of steel primarily increases with an increase in the carbon content. So, the correct answer is: carbon