Nitric oxide reacts slowly with oxygen. The two websites that have a statement contradictory to this aren't as reliable as the other two sources you got them from.
Obviously as you said, this doesn't really matter at all haha.
Anyways, hope that helped.
The question given in your exam, as you might already know, comes straight from the syllabus.
The two conditions you wrote about are perfectly fine and is the reason you got 2 out of the 3 marks. Personally, my textbook does not state the 'metastable' condition, however, there are at least 3...