Which of these is a common type of fuel used in modern boilers?

Natural gas and propane are indeed common fuels for modern boilers due to their efficiency, availability, and relatively cleaner combustion properties compared to traditional fuels. These fuels produce fewer pollutants and greenhouse gases, making them more environmentally friendly options.

Natural gas is often used in residential and commercial applications as it can be piped directly to buildings, offering convenience and reliability in heating systems. Propane, while not as widely available as natural gas, is a viable alternative, especially in areas where natural gas pipelines do not reach. It is typically stored in tanks and is used in a variety of heating applications.

The other options present fuels that are not as commonly used in modern boiler applications. Coal and wood, while historically significant, have declined in usage due to strict emissions regulations and an increasing focus on cleaner energy sources. Nuclear and solar energy, while crucial to the overall energy landscape, are not directly used as fuels in boiler operations but rather as sources of electricity. Kerosene and gasoline are primarily used for transportation and certain heating applications but are generally not utilized in modern boiler systems due to safety and efficiency reasons.