The correct answer is D) closed tightly. When a steam atomizing burner in a steaming boiler is cut out, the register doors for that burner should be closed tightly. This is to prevent air from entering the boiler through the inactive burner, which could disrupt the combustion process and potentially lead to dangerous conditions. Keeping the register doors closed tightly helps to maintain the proper air-fuel ratio and efficient operation of the remaining active burners. The other options are incorrect because leaving the register doors wide open (A) or halfway closed (B) would allow uncontrolled air entry, while leaving them cracked open (C) may not provide sufficient isolation of the inactive burner.