Deterioration of night vision due to hypoxia starts at?
A) 4000
Sea level
C) 8000
D) 10000

There are lots of different numbers depending on the research you read, but most give figures around 4000 to 5000ft when describing the effect of hypoxia on night vision. So A) would be the best guess.

Thank you sir

From personal experience, hypoxic effects on colour and acuity can be quite significant. I recall a chamber run, many years ago, where we were able to play with oxygen supplementation at around 25,000 ft with astounding observation of the effects. Also, in routine airline flying, at the end of a longish night sector, I routinely had a few sucks on the mask during descent with the cabin altitude around, say, 4-5000 feet. On a clear night the lights of the destination brightened up near instantly as if double or treble the lighting was turned on for our viewing pleasure.

Sure, we are permitted to go fly unpressurised at 10,000 feet .. but, in my view, that is not the best idea one can adopt ...