Although boiling point and volatility are both related to the change in phase from liquid to vapor states, boiling only occurs at a fixed temperature when the “vapor pressure” generated by the heated liquid equals/exceeds the atmospheric pressure.
On the other hand, volatility is a qualitative property - a volatile material means it is easily ‘vaporized’. It is quantitatively described using vapor pressure AND boiling point.
High vapor pressures indicate a high volatility
High boiling points indicate low volatility.
So in the context of this question, as number of carbon atoms increases, usually the hydrocarbon changes from gas-> liquid → solid, in which it becomes less volatile DUE TO higher boiling point.
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I understand Thank you very much! 
