Final answer:
The abyssal plains represent the flattest part of the Earth's surface. They are vast, flat areas of the ocean floor covered in sediment and lie between the continental margins and deep sea trenches.
Step-by-step explanation:
The flattest part of the earth's surface is the abyssal plains. Abyssal plains are vast, flat areas of the ocean floor that lie between the continental margins and the deep sea trenches.
They are characterized by very gentle slopes and are covered in thick layers of sediment. Mid-ocean ridges, deep sea trenches, and continental margins are all features of the ocean floor, but they are not as flat as abyssal plains.