The best answer that describes the relationship between the apple tree and the deer is:
C. The deer depends on the tree for apples to eat, and the tree needs the carbon dioxide from the deer to make its own food.
In this scenario, the deer relies on the tree for food in the form of apples, while the tree relies on the carbon dioxide produced by the deer (and other organisms) for photosynthesis, a process crucial for the tree to produce its own food.