Which of the following hydrogen halides is most basic?

The bond strength $\mathrm{H}-X$ decreases from $\mathrm{HF}$ to HI. Thus HF is the most stable and hence weakest acid while is least stable and most acidic. In aqueous solutions, $\mathrm{HF}$ is only slightly ionised but $\mathrm{HCl}, \mathrm{HBr}$ and HI are almost completely ionised. As basicity is the reverse of acidity. Therefore, $\mathrm{HF}$ is the most basic of all $\mathrm{H}-X$.