Ecology: Ecosystems, ecological succession.
A flood is when land that is usually dry gets submerged in water, this can be caused by natural disasters such as hurricanes or typhoons, or after heavy rain when the environment had been modified and can't retain the water.
Floods are not only about the amount of water but also about the force it has. Floods can destroy even houses and bridges, and they left behind sediment dragged from one area to another one.
In an ecosystem, such an amount of water not only drowns the organisms living there but also modifies the characteristics of the soil. Floods are one of the causes of secondary succession, changing the species structure of an ecosystem.
This means the right answer would be C. By killing many organisms and washing away soil