Electric baseboard is not a correct option because it is very expensive.
Forced-air is and hot water are also not correct option because they are less efficient and more expensive in comparison to steam.
Steam as a central heating system is often used when heating many buildings from a central location because it is less expensive and more efficient. For generating steam, solar energy or fossil fuels are used. Steams is passed in the whole buildings through pipelines from a central location.
Steam is often used as a central heating system when heating many buildings froma central location. So the correct option for the given question is option "D". Steam is actually generated ina single room or place mechanically or by useing fossil fuel or solar energy and then it is passed through pipelines to several rooms and buidings. This kind of system is mostly used when heating several rooms or building at once. Electric heaters are normally avoided as the cost of electricity is far higher than any other source of heating.
