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 water in the oceans across the globe changes its characteristics from one area to another area, or from depth to depth. There are multiple factors that contribute to changes in the properties of the water, but the most important one, that tends to heavily influence all other factors, is the temperature of the water.

The oceans, in-depth, by temperature, have a surface zone, thermocline, and deep ocean division. The surface zone is the warmest of the zones, and it is the one where waters with different characteristics mix. The zone below it, the thermocline, is the zone where the water is not mixing anymore. In this zone, the temperature sharply changes between the depth of 200 and 1,000 meters, with every 100 meters of depth meaning 1 degree Celsius less. The deep ocean is the zone that has the biggest depth range, but there isn't much of a change in the temperature and characteristics of the water in it.