Unlike life at the top of a body of water, life at the bottom of the body of water
a receives much more exposure to light
b is affected by temperature and nutrient availability
c performs much more photosynthesis
d must rely on other means than photosynthesis

At the top of the body of a water, there is a lot of light from the sun, which also means a lot of solar energy. This allows for a fruitful photosynthesis.

In contrast, at the bottom of the water, there isn't as much, or sometimes any, sun light. Because of this the organisms at the bottom of the body of  water
D. must rely on other means than photosynthesis

The answer is observations and interpreting.