Answer: [Abyssal plains]
Why: Abyssal plains are flat wide areas of the ocean floor. You can find these features in between a continental rise and a mid-oceanic ridge. These plains are covered in sediment and are found deep down, on the ocean floor. The question, "Which relatively flat feature of the ocean floor is in the open ocean and borders the continental shell?" Directly addresses flat features of the ocean floor. Reading this question and adding what I know to it, I know that my answer is: Abyssal plains.
Vocabulary:
• Abyssal plains - flat wide areas of the ocean floor
• mid-oceanic ridge - a spreading center that bisects the North and South Atlantic basins
• sediment - rock that is formed by the accumulation← (buildup) of small particles
• continental rise - an underwater feature found between the continental slope and the abyssal plain.