Renewable energy refers to energy sources that are naturally replenished and can be used continuously without causing harm to the environment. The correct answer in this case is option 3: biomass.

Option 1, fossils, is incorrect. Fossil fuels such as coal, oil, and natural gas are non-renewable resources that are formed over millions of years and cannot be replenished within a human lifetime.

Option 2, radioactive elements, is also incorrect. Radioactive elements like uranium and plutonium can be used as a source of energy in nuclear power plants, but they are not considered renewable as they have limited reserves and produce harmful waste.

Option 4, natural gas, is a fossil fuel and not a renewable energy source. While natural gas is cleaner-burning than coal or oil, it is still a finite resource that cannot be replenished.

Option 3, biomass, is the correct answer. Biomass refers to organic materials from plants and animals that can be used to produce energy. Examples include wood, agricultural crops, and animal waste. Biomass can be burned directly to produce heat or converted into biofuels like ethanol or biodiesel. It is considered renewable because it can be grown and harvested repeatedly.