The correct answer is B) Radiant-convection. The radiant-convection superheater has the flattest superheat temperature curve because it combines both radiant and convection heat transfer mechanisms. This design allows for a more even distribution of heat, resulting in a more consistent superheat temperature across the superheater section, as compared to other types like pure convection or pure radiant superheaters which can have more variable temperature profiles. The other options are not correct because convection superheaters rely more on airflow patterns, radiant superheaters depend primarily on direct radiation, and conduction-convection designs can have more localized temperature variations.