Which of the following halogen acids is least acidic?

The mixture used for the tips of matchsticks is:

To purify fluorine gas,fumes of $HF$ are removed by

State the occurrence of group-$17$ elements.

In chromyl chloride test,the oxidation state of $Cr$ changes from

Phosphorus reacts with $SO_2Cl_2$ to form $PCl_5$ and $X$. At $723 \ K$,$HCl$ gas reacts with $O_2$ in the presence of $CuCl_2$ to form water and $Y$. $Y$ reacts with $X$ in water to form two acids $A$ and $B$. What are $A$ and $B$ respectively?