the form most stable at a particular temperature of interest and at
standard atmospheric pressure.
(The below won't be on a test; it was added to make things interesting.)
TEMPERATURE OF INTEREST- the often will be "ambient temperature", around
22 deg C. However, if you are working with hypersonic aviation, and
you want to work with a scramjet at speeds above Mach 6, you will be
concerned with temperatures at which molecular oxygen (O2) dissociates
to atomic oxygen, in a reaction that is endothermic, and this is a
problem for you to contend with.
As a hypersonic aviation specialist, you will be considering temperatures
in the other extreme (low temperature) where hydrogen is in a solid
or a "slush" state (I'm paraphrasing what I heard at the AIAA meeting)
so that it will have a much higher density, such that you can store
a lot more of it in a given volume; weight/size constraints are some of
the challenges that hypersonic aviation specialists are investigating.