The correct answer is D) Each heating coil inlet temperature is different, as the hot water inlet temperature to each heating coil progressively drops as the water passes through each successive series-connected coil. In a one-pipe hydronic heating system, the hot water is circulated through a series of heating coils. As the water passes through each successive coil, it loses some of its heat, resulting in a progressively lower inlet temperature for each subsequent coil. This is because the water has already given up some of its heat to the previous coils, so the inlet temperature to each coil is different and decreases as the water flows through the system. The other options are incorrect because they do not accurately describe the temperature behavior in a one-pipe hydronic heating system. Option A is incorrect because the inlet temperatures do not progressively rise, and option B and C are incorrect because the inlet temperatures are not identical.