In the following reactions sequence,
`(A) + N_(2) overset(Delta)(to) (B) overset(+H_(2)O)(to) (C ) + (D)`
White ppt. (C ) is formed and gas (D) is evolved. White ppt. (C ) dissolves in NaOH solution, while gas (D) gives white fumes in HCI. Thus, (A) is

Explanation

Metal `(A)` on heating with nitrogen, `N_(2)`, gas gives `(B)` which on hydrolysis gives a ppt. `( C)` and a gas `(D)`. Since precipitate `(C )` dissolves in `NaOH` solution, metal `(A)` must be aluminium, `Al`. As the gas `(D)` gives white fumes with `HCl (D)` is ammonia `(NH_(3))`.
`underset((A)) (2Al) + N_(2) overset(Delta) underset(underset("Aluminium nitride")((B))(2AlN)`.
`underset((B))(AlN) + H_(2) O rarr underset(underset(underset(underset(("white ppt."))("hydroxide"))("Aluminium"))((C)))(Al(OH)_(3) darr) + underset(underset("Ammonia")((D)))(NH_(3)uarr_((g)))`
`Al(OH)_(3) + NaOH_((aq)) rarr underset(underset(underset(("Soluble"))("metaluminate"))("Sodium"))(NaAlO_(2)) + 2H_(2) O`
`underset((D)) (NH_(3)) + HCl rarr underset(("White fumes"))(NH_(4)Cl)`
Hence `(A)` is `Al , (B)` is `AlN , (C)` is `Al(OH)_(3)` and `(D)` is `NH_(3)`.