Answer:
C. Fission
Step-by-step explanation:
Geothermal energy is created by the Earth's core, and tidal energy is created by the ocean. We can eliminate these answers immediately.
Fusion occurs when unstable smaller isotopes, chiefly deuterium and tritium, fuse. Hence 'fusion'.
This leaves us with fission, which is the process of a large and unstable isotope's nucleus, such as that of Uranium-235, splitting apart into daughter nuclei. This creates a reaction that causes them to fizz out, so that's how I remember that splitting is fission.