The correct answer is B) water-tube forced circulation. Auxiliary boilers are divided into several classifications, one of which is the water-tube forced circulation type. In this design, the water circulates through the boiler tubes due to forced circulation, such as by a pump, rather than relying on natural convection. This forced circulation allows for more efficient heat transfer and can help prevent overheating or damage to the boiler. The other answer choices are incorrect because A) water-tube supercritical circulation refers to boilers operating at pressures above the critical point of water, C) fire-tube controlled circulation uses a different type of boiler design, and D) fire-tube express circulation is not a standard classification for auxiliary boilers.