Which of the following hydrogen halide is most volatile?

Boiling point of $\mathrm{HF}$ is highest due to $\mathrm{H}$-bonding. For other halogen acids boiling point increase in the order $\mathrm{HCl}<\mathrm{HBr}<\mathrm{HI}$. Therefore, most volatile (with lowest boiling point) is $\mathrm{HCl}$.