Final answer:
The term that defines a wide, flat area that can be covered by water during floods is 'flood plain.'
Step-by-step explanation:
The word that best fits the definition of a wide, flat area covered by water during flooding is b. flood plain. A flood plain is an area of land that is prone to flooding, typically situated near a river or stream. When the river overflows during periods of high water, such as during heavy rainfall or snowmelt, it spills out over the flood plain, covering it with water. This natural occurrence is essential for creating fertile soils due to the deposition of sediments, and some cultures, like the ancient Egyptians, heavily relied on these flood plains for agriculture.