Anion which can react with $dil. H_2SO_4$ is

Given below are two statements:
Statement $I$: Griess-Ilosvay test is used for the detection of nitrite ion, which involves the use of sulphanilic acid and $\alpha$-naphthylamine reagent.
Statement $II$: In the above test, sulphanilic acid is diazotized by the acidified nitrite ion, which on further coupling with $\alpha$-naphthylamine forms an azo-dye.
In the light of the above statements, choose the correct answer from the options given below.

$A$ salt gives violet vapours when treated with conc. $H_2SO_4$. It contains

The $Chromyl$ $Chloride$ test is performed to detect which of the following ions?

$A$ sodium salt gives a white precipitate with $MgCl_2$ only upon heating. What is the anion of the sodium salt?

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