For these questions, we are trying to identify the answer choice that provides a piece of information that allows the two seemingly incompatible items/events to occur together.
In this case, the paradox is how is it possible that mule deer who live in a cleared forest, in which their food source grows better, are more malnourished than are mule deer who do not live in a cleared forest and thereby do not have abundance of their food source?
Answer choice D answers this quite nicely. It is important to remember that the first premise given was that cleared forests receive more sunlight on the forest floor. If more sunlight results in more tannin, which inhibits digestion of plant protein, then this would perfectly explain how despite the abundance of this plant, these deer are malnourished; they cannot digest the plant as well!
Answer choice E doesn't explain why deer with more available food are less nourished. In fact, it is confusing because if ticks living in trees come down from the trees to feed on deer, then it is preferable for the deer to be in cleared forests. This is just providing another reason why it should be good to live in a cleared forest, and adds nothing to explain why malnourishment results in the face of abundance.
I hope this helps. Please let me know if you have any other questions!