Planet earth is the only known region of the universe to host life and so the basic criteria for life is based on observations made here. For an entity to be considered a living organism it must contain DNA or some other means of storing genetic information. The second criteria is that it must have some way of extracting nutrients from its surroundings. The organism must be able to respond to environmental stimulus and last but not least it must be able to reproduce.
The answer is D, photosynthesis and respiration.
—Evidence—
Oxygen is a byproduct of photosynthesis and, correspondingly, carbon dioxide the byproduct of respiration.
Oxygen gives our cells the ability to break down food in order to get the energy we need to survive. Although other animals may use different organs to breathe with, they all get oxygen into their bodies through respiration.
Protein synthesis as well as DNA transcription
