The correct answer is A) Combustion gases flow through the tubes. In a fire-tube boiler, the hot combustion gases from the furnace flow through the tubes, which are surrounded by water. This allows the heat from the gases to transfer to the water, generating steam. The tubes act as heat exchangers, allowing the heat from the combustion process to be efficiently transferred to the water. The other options are incorrect because: B) Flames do not impinge on the tubes, but rather the hot gases flow through them; C) Combustion occurs in the furnace, not inside the tubes; and D) it is the combustion gases, not the water, that flow through the tubes in a fire-tube boiler.