1. Identify gas $B$ from its reaction with $CuSO_4$

   The gas $B$ reacts with aqueous $CuSO_4$ to form a deep blue compound. This is characteristic of ammonia, because ammonia forms the deep blue tetraammine copper(II) complex: $Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}$

   So, $B = NH_3$

2. What kind of nitride gives $NH_3$ with water?

   Metal nitrides react with water to produce ammonia. For a nitride of formula $M_3N$: $M_3N + 3H_2O \rightarrow 3MOH + NH_3$

   Hence the given nitride must be such that its hydrolysis gives ammonia.

3. Check which metal forms nitride of formula $M_3N$

   Let the oxidation state of $M$ be $+1$, since nitride ion is $N^{3-}$ and the formula is $M_3N$.

   So $M$ must be an alkali metal.

   Among the options, possible choices are:

   • Li gives $Li_3N$
   • Na would have to give $Na_3N$, but sodium does not readily form a stable nitride like lithium does.

4. Use the thermal decomposition clue

   The compound $M_3N$ on strong heating gives back the metal:

   $$$$

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Lithium is the alkali metal known to react directly with nitrogen to form a stable nitride: $6Li + N_2 \rightarrow 2Li_3N$

5. Check hydrolysis of lithium nitride

   $Li_3N + 3H_2O \rightarrow 3LiOH + NH_3$

   Thus the gas evolved is indeed: $B = NH_3$

6. Evaluate options

   • A: Li and $NH_3$ — correct
   • B: Ba and $N_2$ — wrong, gas $B$ should be $NH_3$, not $N_2$
   • C: Na and $NH_3$ — wrong, sodium does not form the corresponding stable nitride in this standard way
   • D: Al and $N_2$ — wrong, $AlN$ is formed, not $Al_3N$, and with water it gives $NH_3$, not $N_2$

7. Final answer

   Therefore, $M = Li, \quad B = NH_3$

   So the correct option is A.