c) In dense rainforest, plants that grow close to the forest floor tend to have lower rates of photosynthesis than the very tall trees. Suggest why this might be.

The reason why plants that grow close to the forest floor tend to have lower rates of photosynthesis is because sunlight is a vital component in the production of glucose. Using logic, you can see that the tree cause larger shadows to form—blocking sunlight.  

i believe the correct answer is d