A metal ‘M’ reacts with nitrogen gas to afford‘M₃N’. ‘M₃N’ on heating at high temperaturegives back ‘M’ and on reaction with waterproduces a gas ‘B’. Gas ‘B’ reacts withaqueous solution of CuSO₄ to form a deep blue compound. ‘M’ and ‘B’ respectively are 
 

Li and NH₃​
The metal ‘M’ forms nitride M3​N. Since N has valency 3, metal ‘M’ has valence 1. Hence, ‘M’ can be either Li or Na. So we can rule out Ba and Al.

M₃​N reacts with water to produce gas ‘B’. This is given by Li
Li₃​N+3H₂​O→3LiOH+NH_3​
Thus, gas ‘B’ is ammonia

Ammonia reacts with aqueous CuSO_4​ solution to form deep blue compound [Cu(NH₃​)₄​]SO_4​
NH₃​+CuSO₄​→[Cu(NH₃​)_4​]SO_4​