(b)
The formula of $'A^{'}is\ M_{3}N.$ It suggests that $M$ is a monovalent metal.
$\ \ \ \ \ \ \ \ \ 6Li + N_{2}\ \rightarrow 2Li_{3}N\ \overset{\ \ \ \ \ \mathrm{\Delta}\ \ \ \ \ }{\rightarrow}\ 6\ Li + N_{2}$
$\ \ \ \ \ \ \ \ \ (M)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (A)$
lithium nitride
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ Li_{3}N + 3H_{2}O\ \ \rightarrow 3LiOH + NH_{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (B)$
$\ \ \ \ \ \ \ \ \ \ \ \ CuSO_{4} + 4NH_{3}\ \rightarrow \left\lbrack Cu\left( H_{3} \right)_{4} \right\rbrack SO_{4}$
blue solution
Hence, $M\ and\ B$ are Li and $NH_{3}$ respectively.