Answer : 4. "geothermal energy"
Explanation :

Answer: D) geothermal energy Explanation: Based on conventionality in deriving energy, energy sources could be classified as   Conventional :: coal, oil, hydro, nuclear, etc... sources which can be used up or exhaustible. (and) Non - conventional :: solar, wind, tidal, geothermal, biogas, etc... sources which are not exhausted.   Among the given options, Geothermal energy is the alternative energy source among all the choices that are given in the question, that is exhaustible.