Which term describes the deep flat part of the ocean floor?

The correct answer is abyssal plain.  

The ocean basin or the abyssal plain refers to the flat region of the ocean floor. It is enveloped with mud, sand, and animal and plant remain. Situated on this flat plain are underwater mountains known as seamounts, which are produced by volcanic eruptions.  

The challenger deep, in the southern terminal of the Mariana Trench is the deepest point in the ocean.  

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