The correct answer is (d), solar cells. Solar cells are a type of renewable energy source that converts sunlight into electricity. They are clean and do not produce any emissions, making them a good choice for keeping the environment clean.
Petroleum products are a non-renewable energy source that is derived from fossil fuels. They are a major source of air pollution and greenhouse gases, which contribute to climate change.
Forest products are a renewable energy source, but they can be harmful to the environment if they are not harvested sustainably. Deforestation can lead to soil erosion, loss of biodiversity, and climate change.
Nuclear fission is a non-renewable energy source that produces radioactive waste. This waste can be harmful to the environment if it is not properly disposed of.
In conclusion, solar cells are the best energy source for keeping the environment clean. They are clean, renewable, and do not produce any emissions.