A fireball is a spherical, buoyant flame that forms when a cloud of fuel vapor ignites. The interior is rich in fuel vapor, while the outside experiences combustion where air can mix with fuel at the flame front. Because the reaction concentrates at the outer edge, the flame appears to burn from the outside inward toward the center. The hot gases rise quickly, giving the flame upward momentum. It tends to be relatively short-lived because the available fuel-air mixture is consumed rapidly and the conditions necessary for continued burning dissipate. The other options don’t describe this spherical, brief, buoyant flame pattern. A solid-fuel fire tends to burn at the surface and not as a self-contained ball, a jet flame is elongated and fed by a continuous stream, and “fuel” as a general category doesn’t capture the distinct ball-like, edge-to-center burning behavior.