Until the late 1800s, which of the following sources was used to heat most homes?

fossil fuels

electricity

biomass

solar energy

Fossil fuels because back then they didn't have technology for solar energy and electricity. Then biomass was used after the 1800s.

The correct answer is option C, Biomass

Explanation:

Fossil fuels was discovered in year 1600 but since it was not mine and produced commercially till later 1700, thus it could not be a major source of heat till 1800.

Also the first electric bulb was found in 1879 by Thomas Edison and the first solar panel was installed in 1839 by Alexandre Edmond

Thus till 1800, it was biomass majorly that was considered a good source of heat.