One alternative to burning non-renewable fuels, which harm the environment, could be to burn?
a biomass
b coal
c natural gas
d petroleum

The correct answer is A biomass.

Explanation:

Biomass is plant or animal material used for energy production (electricity or heat), or in various industrial processes as raw material for a range of products. It can be purposely grown energy crops (e.g. miscanthus, switchgrass), wood or forest residues, waste from food crops (wheat straw, bagasse), horticulture (yard waste), food processing (corn cobs), animal farming (manure, rich in nitrogen and phosphorus), or human waste from sewage plants.

Burning plant-derived biomass releases CO2, but it has still been classified as a renewable energy source in the EU and UN legal frameworks because photosynthesis cycles the CO2 back into new crops. In some cases, this recycling of CO2 from plants to atmosphere and back into plants can even be CO2 negative, as a relatively large portion of the CO2 is moved to the soil during each cycle.

grav potential energy

it has to be b, about 97 percent.