Renewable energy means that there is an unlimited supply. No matter what happens it won't be destroyed or ever go away/run out. This means the answer can only be natural gas. Natural gas is formed from reallyyyyyyyyy old, dead organisms, and takes a looooooooong time to form (think millions of years). Because it takes a long time to form it won't be guaranteed that natural gas will always be there like water, wind, and heat from the earth (geothermal) .
Hope this helped!
