Which of the following ions forms a white precipitate upon the addition of $HCl$ to its aqueous solution?

Salt $X$ is:
$X \text{ (Colourless salt)}$ $\xrightarrow[\Delta]{NaOH} Y$ $\xrightarrow{HCl} \text{White fumes}$
$X \text{ (Colourless salt)}$ $\xrightarrow{CaCl_2} Z \text{ (White ppts.)}$ $\xrightarrow{KMnO_4} \text{Colourless solution}$

$A$ is:
$\text{Salt } A \text{ (Colourless)} \xrightarrow{Cu} \text{Blue solution}$
$\text{Salt } A \text{ (Colourless)} \xrightarrow{HCl_{(aq)}} \text{White ppt}$
$\text{Salt } A \text{ (Colourless)} \xrightarrow{\text{Conc. } H_2SO_4} \text{Brown fumes}$

Which ion is detected using Nessler's reagent?

Reagents used for the qualitative estimation of $Ni^{+2}$ and $Cu^{+2}$ respectively are:

$A$ solution is prepared by mixing a concentrated solution of $Co(NO_3)_2$ with a concentrated solution of $NaNO_2$ in $50\%$ acetic acid. $A$ solution of a salt containing metal $M$ is added to the mixture, and a yellow precipitate is formed. Metal $M$ is: