Question

which atmospheric gas crystallizes in the upper atmosphere and creates visible clouds? methane oxygen water vapor carbon dioxide

Explanation:

Water vapor in the upper atmosphere can cool and condense or crystallize (as ice crystals in cold enough conditions) to form visible clouds. Methane, oxygen, and carbon dioxide do not typically form visible clouds in the same common and prominent way as water vapor does in the context of general atmospheric cloud - formation we observe.

Answer:

water vapor