Which is the most volatile compound?

Which one of the following halogens liberates oxygen when passed through hot concentrated $KOH$ solution?

Compound $A$ and $B$ respectively are :-
$Cl_2 + 2NaOH (cold \& dil.) \rightarrow A + NaCl + H_2O$
$3Cl_2 + 6NaOH (hot \& conc.) \rightarrow B + 5NaCl + 3H_2O$

When $MnO_2$ and $H_2SO_4$ are added to $NaCl$,the greenish-yellow gas liberated is:

Explain the electron gain enthalpy values of elements of the $17^{\text{th}}$ group. Give reasons and explain.

The least active halogen with hydrogen is