The correct answer is C. Properly functioning steam traps should prevent the backup of condensate in the steam system, thus preventing the loss of sensible heat and the occurrence of "water hammers" without passing steam. Steam traps are essential components of an efficient steam system, as they help maintain the proper pressure and temperature by removing condensate from the steam lines. By preventing the buildup of condensate, steam traps help ensure that the steam system operates efficiently, preventing the loss of energy and potential damage from water hammers. The other options are incorrect because they do not accurately describe the primary function of properly functioning steam traps. Option A is incorrect as steam traps are not designed to release trapped air to the atmosphere. Option B is incorrect as steam traps should prevent the backup of condensate, not allow it. Option D is incorrect as steam traps should not allow both steam and condensate to pass through, as this would lead to inefficient operation.