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