Answer: B (neon). When the cooled air is warmed up, each liquefied substance boils and turns into a gas as soon as the temperature reaches its boiling point. The substance with the lowest boiling point is reached first.
The boiling points are neon −246 °C, nitrogen −196 °C, argon −186 °C and oxygen −183 °C. Neon has the lowest boiling point (−246 °C), so as the temperature rises from −250 °C it is the first to boil and become a gas.
Oxygen (−183 °C) would be the last to turn into a gas. This is a change of state: a substance changes from liquid to gas (boils) at its boiling point.
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