What is the thermocline? beneath 200 meters, ocean water cools at a constant rate of 1°C per 100 meters depth
the ocean area near the polar regions where water temperatures cool
where magma is forced up through the ocean floor along mid-ocean ridges
the area off the continental shelf that declines to the ocean floor

Beneath 200 meters, ocean water cools at a constant rate of 1°C per 100 meters depth.

Explanation:

The thermocline is one of the three ocean zones when it comes to separating the ocean waters by temperature and weather waters mix or not. The top layer of water, or the surface water, is called the epilimnion. The second layer, or thermocline, is called metalimnion at starts from 200 meters of depth. And the last one, or deep ocean, is the hypolimnion, starting from 1,000 meters of depth.

The surface layer is the one where the water is the warmest and it mixes. In the second layer, we have the thermocline, where the water is not mixing anymore and it sharply drops in temperature. The water temperature generally drops by 1 degree Celsius per 100 meters of depth. The third layer is the coldest, but there aren't big changes in temperatures between the starting and endpoint of it.