The brown ring test is used to detect the presence of which ion?

Among the following observations, the correct one that differentiates between $SO_{3}^{2-}$ and $SO_{4}^{2-}$ is:

On addition of conc. $H_2SO_4$ to a chloride salt,colourless fumes are evolved,but in the case of an iodide salt,violet fumes are observed. This is because:

$M \text{ (Salt)} + \text{Dil. } HCl \xrightarrow{\text{Warm}} N \uparrow + P \downarrow$. Gas $N$ changes the colour of $FeSO_4$ solution into a yellow solution. Then,salt $M$ in the above reaction is:

$A$ salt solution is treated with chloroform drops. Then it is shaken with chlorine water. The chloroform layer becomes violet. The solution contains:

Chromyl chloride vapours are dissolved in $NaOH$ and acetic acid and lead acetate solution is added,then