The least active halogen with hydrogen is

Bromine is commercially prepared from sea water by displacement reaction:
$Cl_2 + 2Br^{-} (aq.) \to 2Cl^{-}(aq.) + Br_2$
$Br_2$ gas thus formed is dissolved into a solution of $Na_2CO_3$ and then pure $Br_2$ is obtained by treatment of the solution with:

Which of the following properties does not correspond to the order $HI < HBr < HCl < HF$?

The correct order of acidic strength is:

Among the following halogens $F_2$,$Cl_2$,$Br_2$ and $I_2$,which can undergo disproportionation reaction?

$A$ solid $A$ which has photographic effect reacts with the solution of a sodium salt $B$ to give a pale yellow precipitate $C$. The sodium salt $B$ on heating with concentrated $H_2SO_4$ gives brown vapours. Identify $A$,$B$,and $C$.