ANS:D - 20

The minimum thermal efficiency of a steam boiler may be around 20 percent. Explanation: The thermal efficiency of a steam boiler represents the ratio of the useful output (heat) to the input (fuel energy) and is typically expressed as a percentage. It is calculated using the following formula: Thermal efficiency=InputUseful /output​×100% For steam boilers, the useful output is typically the heat energy transferred to the steam, which is used for various applications such as heating, power generation, or industrial processes. The input is the energy content of the fuel used to produce the steam, typically measured in terms of heat content or chemical energy. The minimum thermal efficiency of a steam boiler depends on various factors such as the design, size, operating conditions, and fuel type. However, modern steam boilers are generally designed to achieve high thermal efficiencies to maximize energy conversion and minimize fuel consumption and environmental impact. While the exact minimum thermal efficiency may vary depending on specific circumstances, a typical minimum thermal efficiency for a well-designed and properly operated steam boiler is around 20 percent. This means that at least 20 percent of the energy content of the fuel is effectively transferred to the steam, with the remaining energy lost as waste heat through various mechanisms such as flue gas exhaust, radiation, and unburned fuel. Therefore, among the options provided, the minimum thermal efficiency of a steam boiler may be around 20 percent.