I would go with mathmatical formula because of physics and what not but I'm not completely sure so if you have a different possible answer you think would be better, go with that :)
Answer:
When the Earth Rotates on its axis.
Explanation:
That is how the whole day/night cycle on earth progresses, is by the earth rotating on its axis. None of the others would work.
Interactions may be commensalism, mutualism, or parasitism.
The answer is a cotyledon