Which term refers to broad, smooth regions in the deep-ocean basins on either side of an oceanic ridge?

The term that refers to broad, smooth regions in the deep-ocean basins on either side of an oceanic ridge is the abyssal floor. The abyssal floor is part of the ocean's deep-sea environment, characterized by depths typically ranging from about 3,000 to 6,000 meters. These areas are often flat and covered with fine sediment, making them distinct from other ocean features.

The abyssal floor is important in understanding the geology and biology of the ocean, as it hosts unique ecosystems and serves as a setting for sediment deposition and geological processes. Features such as mid-ocean ridges, which are underwater mountain ranges, are surrounded by these flat plains, highlighting the relationship between various oceanic features.

The other options, such as continental shelf and coastal plains, refer to shallower areas closer to land, while mid-ocean trenches are deep, narrow valleys in the ocean floor, not smooth expanses. This positioning of the abyssal floor relative to geological structures like oceanic ridges underscores its significance in marine geology.