The least harmful among the three would be geothermal energy. Though all of them are renewable resources, biofuel and biomass can cause more environmental effects than geothermal energy. In order to obtain energy from biofuel and biomass, the process of burning these materials can emit harmful substances in the environment which can contribute to air pollution. On the other hand, geothermal energy is more environmentally friendly compared to the two because its pollution level is much lower.
