A rainforest is humid, therefore, water uptake would likely not be an issue. However, rainforests have a huge number of trees, often these are very tall. These large trees might block out the sun from the smaller plants nearer the ground. This means they would struggle to properly photosynthesise, as they wouldn't have the necessary light energy.
Therefore, as a survival adaptation, it is likely that an allele that produced broader leaves for photosynthesis would be selected for in this population. Broader leaves would give a larger surface area for the plant to absorb light energy to perform photosynthesis.
The factor that has the most greatest importance in determining the characteristics of an aquatic ecosystem will be water depth. The depth will determine the sunlight that the ecosystem will get. The answer will be B.