Final answer:
The correct order of grades of coal from lowest energy content to highest energy content is sub-bituminous, bituminous, lignite, anthracite.
Step-by-step explanation:
The correct order of grades of coal from lowest energy content to highest energy content is:
c) Sub-bituminous, bituminous, lignite, anthracite
Coal is classified into four main types, including anthracite, bituminous, subbituminous, and lignite. Anthracite is the highest grade of coal and has the highest energy content, while lignite has the lowest energy content.