The correct answer is C) an atmospheric drain tank trap frozen in the closed position. An atmospheric drain tank trap that is frozen in the closed position can cause excessive water loss from the main feed system. This is because the trap is unable to properly drain the condensate from the tank, leading to a buildup of water and potential loss from the system. The other options are incorrect because: A) excessive recirculation of condensate would not directly cause water loss, B) a leak in the desuperheater gasket would not affect the main feed system, and D) a vapor bound main condensate pump would not lead to excessive water loss.