Final answer:
The ocean floor region, formed from eroded materials from the continental shelf and slope, that flattens out at the base of the continental shelves is called the abyssal plain.
Step-by-step explanation:
At the base of many continental shelves, the ocean floor flattens out to a broad region formed from materials eroded from the adjoining continental shelf and slope. This plain is termed the (b) abyssal plain.
The continental shelf is the submerged portion of a continent's crust in relatively shallow water, extending from the shoreline to the point where the seafloor begins to steeply drop off at the shelf break. Beyond this, the continental slope leads down to this flat, extended region known as an abyssal plain, which is comprised predominantly of fine sediments that have settled over time.
Abyssal plains are typically found at depths of about 4,000 to 6,000 meters below the sea surface and represent some of the most level natural surfaces on Earth. They are formed by the slow accumulation of sediment, brought to these depths by gravity-driven processes like turbidity currents which wash sediment from the continental shelf and slope. These deposits create the abyssal plain, essentially blanketing the ocean floor with a layer of sediment.