If you are flying under the anvil of a thunderstorm, what is the main hazard?

When flying under the anvil of a thunderstorm, the main hazard is that hail can be discharged from the anvil. Thunderstorms often produce hail, especially in stronger storms, and the anvil portion can serve as a zone where hail is released. This hail can be large, posing a significant danger to aircraft, as it may cause structural damage or lead to loss of control if ingested into the engine.

While severe turbulence is indeed a concern associated with thunderstorms and can occur generally beneath a storm, the specific threat of hail beneath the anvil is particularly notable since it is directly linked to the anvil structure of the thunderstorm. Lightning strikes are also a risk in proximity to thunderstorms, but the likelihood is more about the overall storm environment rather than being directly under the anvil. Rain falling may not be harmful, but it doesn't capture the critical hazard posed by the potential for hail from the anvil area. Thus, the correct answer emphasizes the significant risk of hail as a danger for aircraft operating in that area.