The correct answer is A) steam demand response is comparatively rapid. In a coil-type forced circulation auxiliary water-tube boiler, the steam demand response is comparatively rapid because the heating coils are designed to quickly transfer heat from the combustion gases to the water, allowing the boiler to respond quickly to changes in steam demand. This rapid response is a key advantage of this boiler design. The other options are incorrect because B) steam is not recirculated through heating coils (it flows through the coils once), C) unevaporated feed water is not discharged through the skim tube (the skim tube removes impurities), and D) the steam demand response is not slow, as explained in the correct answer.