Animals use oxygen for cellular respiration.
Plants release oxygen during photosynthesis, but use it during cellular respiration.
Bacteria containing nucleic acids
The first box is migration
The second box is the northern regions have become warmer because of climate change
The answer here is D, and it's kind of obvious. But you probably already knew the answer and are just doing this for piece of mind.
Explanation: Just the fact that we don't know whether or not there even is extraterrestrial life rules out every other answer, without even needing to go in depth.
(Okay, I read your question completely wrong, it's asking for which one of these has been solved isn't it?)
This is tough because all of these other than D have been figured out. Go with the methane atmosphere then.
