Which of the following diazonium salts $RN_2^+X^-$ will be the most stable?

The correct statements of the following are:
$A$. Aniline forms a stable benzene diazonium chloride at $285 \ K$.
$B$. $N$-Phenylethanamide is less reactive towards nitration than aniline.
$C$. $p-CH_3C_6H_4COCl$ is Hinsberg reagent.

The increasing order of $pK_b$ values of the following compounds is

Account for the following:
$(i)$ $pK_{b}$ of aniline is more than that of methylamine.
$(ii)$ Ethylamine is soluble in water whereas aniline is not.
$(iii)$ Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
$(iv)$ Although amino group is $o-$ and $p-$ directing in aromatic electrophilic substitution reactions,aniline on nitration gives a substantial amount of $m$-nitroaniline.
$(v)$ Aniline does not undergo Friedel-Crafts reaction.
$(vi)$ Diazonium salts of aromatic amines are more stable than those of aliphatic amines.
$(vii)$ Gabriel phthalimide synthesis is preferred for synthesising primary amines.

The sequence of reagents which convert $p-$methyl aniline to $p-$methyl benzoic acid are

Identify $Z$ in the above sequence of reactions.