The correct answer is C) To allow access into the headers. Handholes on a boiler are primarily used to provide access to the headers, which are the large pipes that connect the steam and water drums to the rest of the boiler system. This allows for inspection, maintenance, and cleaning of the internal components of the boiler, ensuring safe and efficient operation. The other answer choices are incorrect because: A) Access to the steam and water drum is typically provided through manholes, not handholes. B) Cleaning the firebox is done through the furnace door, not the handholes. D) Accessing the stack is not the primary function of handholes on the boiler itself.