Which term is described as a flat expanse of ocean floor a) guyots b) abyssal plains c) continental shelf d) deep- sea trenches

The answer is B: abyssal plains

Abyssal plain is the term that is used described as a flat expanse of ocean floor.

Explanation: Abyssal plains refer to featureless areas on earth which are flattest, huge and sediment-covered area of the deep ocean floor. Abyssal plain are found at depths between 3,000 metres (9,800 ft) and 6,000 metres (20,000 ft).  Abyssal plains cover more than 50% of the earth surface.