The correct answer is B) fire-tubes and stay-tubes. In a fire-tube auxiliary boiler, the tube sheets are connected by the fire-tubes and stay-tubes. The fire-tubes carry the hot gases from the furnace through the boiler, while the stay-tubes provide structural support and stability to the tube sheets. This arrangement is a key design feature of fire-tube boilers, as it allows for efficient heat transfer from the hot gases to the water within the boiler. The other options are incorrect because: A) girder stays are not used to connect the tube sheets in a fire-tube boiler, C) the external boiler plating does not directly connect the tube sheets, and D) separate crown sheets are not a common feature of fire-tube auxiliary boilers.