The correct answer is C) Alternately fire one burner for 15 minute intervals during a 5 hour period. This is the correct procedure to dry out the fireside of a boiler after water washing. Alternately firing one burner for short intervals helps to gradually and evenly dry out the surfaces inside the furnace, preventing any sudden temperature changes that could damage the boiler. The 5 hour time period ensures the fireside is thoroughly dried before returning the boiler to normal operation. The other options are incorrect because: A) using silica gel is not an effective way to dry out a boiler furnace, B) using superheated steam could potentially damage the boiler, and D) simply running the fans without controlled firing would not properly dry the surfaces.