The answer here is D: ATP. Let's look first at a simplified chemical equation for respiration:
C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy
While the actual reaction pathway with all of the intermediate steps is much more complicated, the above equation is the ultimate end result. Glucose and oxygen are converted to carbon dioxide, water, and energy. The energy is stored in the form of ATP. Now let's look at the multiple choice answers in turn:
A is incorrect because glucose is a starting material, not a product.
B is incorrect because CO2 is simply exhaled from the body through the lungs. It is not an important product; it is more like a byproduct.
C is incorrect because oxygen is a starting material, not a product.
D is correct. Producing energy in the form of ATP is the ultimate reason that respiration occurs.
E is incorrect because water is more like a byproduct. The amount of water taken into the body through eating and drinking is much more significant than the water produced in respiration.