Half of the challenge behind piloting exams is understanding what the question is asking. I think this is one example of many poorly-worded questions. AFAIK hypoxia doesn't care what time of day it is, it's all about absorbing oxygen. However, some of the symptoms of hypoxia include reduced colour and night vision, which is bad news for night flight.
If the question doesn't make sense, look at the answers for clues. Out of the 3 options here, 2 of them are very similar - times where it is nighttime year round, and one where it is daylight (varying with latitude and season of course).
So the exam is asking you to choose between day and night, essentially. The symptoms of hypoxia are more abundant at night, so I think you have your answer there.