The correct answer is (b) Abyssal plains
The flat expanse of ocean floor at the bottom of the sea is called abyssal plain which accounts for about 79% of the sea floor worldwide. It is the smoothest surface on the planet with less than 5 feet of vertical variation, that is most common in Atlantic.
Example: Largest Abyssal plain is Canary Abyssal
the correct answer is abyssal plains. Hope this works!