The correct answer is B) circulation. Overheating of the generating tubes in a boiler occurs when the boiler reaches the end point of its circulation. When the circulation of water and steam within the boiler becomes impaired, certain areas of the boiler, particularly the generating tubes, can become overheated. This is because the reduced circulation prevents adequate cooling and heat transfer, leading to the overheating of the generating tubes. The other options are incorrect because they do not directly relate to the root cause of overheating in the generating tubes. Combustion (A), moisture carryover (C), and evaporation (D) are related to other boiler processes, but they do not specifically address the issue of impaired circulation leading to overheating.