Answer:
The correct answer is option C, Biomass
Step-by-step 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 .