The correct answer is C) boiler water carryover. Boiler water carryover can lead to the formation of damaging scale on the interior of superheater tubes. When water is carried over from the boiler into the superheater, the dissolved minerals and solids in the water can deposit and accumulate as scale on the superheater tube surfaces. This scale buildup can insulate the tubes, leading to higher temperatures and accelerated degradation of the tube material. The other answer choices, such as high superheater outlet temperature or insufficient steam flow, can contribute to superheater issues, but they do not directly cause the formation of damaging scale like boiler water carryover does.