The correct answer is - a.) geothermal.
The geothermal energy is a natural resource, and it is considered as part of the renewable energy sources. Even though it is an energy source that is environment friendly, the geothermal energy has a decline in its usage in the recent years. This is due to its instability in providing energy, as well as having other renewable energy sources that are more efficient than the geothermal energy. Also, a part in its declining usage as an energy source has the tourism sector, as the hot water stuffed with lots of minerals is very attractive for the tourists.