The correct answer is: A. Light is absent.
Green plants need sunlight to photosynthesize, which is the process by which they convert sunlight into energy. The deeper you go into the ocean, the less sunlight there is. At depths of about 200 meters, there is no sunlight at all. This is why green plants are absent in the deep sea.
Option B, high pressure, is not the correct answer because green plants can survive in high pressure environments. For example, kelp forests can grow in water that is up to 200 meters deep.
Option C, low temperature, is not the correct answer because green plants can survive in cold environments. For example, some algae can grow in water that is below freezing.
Option D, great depth, is not the correct answer because green plants can survive in deep water. For example, some algae can grow in water that is over 1,000 meters deep.
However, green plants cannot survive in complete darkness. This is why they are absent in the deep sea, where there is no sunlight.