Answer:
Natural gas
Step-by-step explanation:
Nuclear is from splitting atoms in a nuclear power plant and doesn't emit GHG.
Hydroelectric is a clean energy which powered by water currents in turn not emitting any GHG.
Geothermal is heat captured from underneath the surface and within Earth and doesn't have GHG once the contruction of putting pipes underground is done.
Natural gas once burned emits GHG.