Which reagent can be used to identify nickel ion $(Ni^{2+})$?

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$(NH_4)_2SO_4 + Sr(OH)_2 \longrightarrow SrSO_4 \downarrow + 2NH_3 \uparrow + 2H_2O$

$A$ colorless aqueous solution contains nitrates of two metals,$X$ and $Y$. When it was added to an aqueous solution of $NaCl$,a white precipitate was formed. This precipitate was found to be partly soluble in hot water to give a residue $P$ and a solution $Q$. The residue $P$ was soluble in aqueous $NH_3$ and also in excess sodium thiosulfate. The hot solution $Q$ gave a yellow precipitate with $KI$. The metals $X$ and $Y$,respectively,are

The $V^{-}A$ group precipitate was dissolved in $HNO_3$ and treated with an excess of $NH_4OH$. It gives a white precipitate because of:

Yellow ammonium sulphide solution is a suitable reagent for the separation of

The colour of $Mg(OH)_2$ precipitate formed when it is precipitated in the presence of magneson reagent is