Regarding the chemistry of nitrogen, phosphorus and boron compounds, the correct statement(s) is/are

(A) $\mathrm{Mg_3N_2+6H_2O\rightarrow3Mg(OH)_2+2NH_3}$: the nitride ion is a strong base and is protonated by water, so the statement is correct. (B) Boric acid is a weak monobasic Lewis acid: it accepts $\mathrm{OH^-}$ from water to form $\mathrm{[B(OH)_4]^-}$ and releases only one proton; the three $\mathrm{O{-}H}$ bonds are not ionised, so (B) is wrong. (C) In the solid state $\mathrm{PCl_5}$ is ionic, $\mathrm{[PCl_4]^+[PCl_6]^-}$, with $sp^3$ phosphorus in the cation (tetrahedral) and $sp^3d^2$ phosphorus in the anion (octahedral): correct. (D) $\mathrm{H_3PO_3}$ has the structure $\mathrm{HPO(OH)_2}$, with one P{-}H bond that is not acidic, so it is dibasic; on heating, $4\mathrm{H_3PO_3}\rightarrow3\mathrm{H_3PO_4}+\mathrm{PH_3}$, a disproportionation of P(+3) to P(+5) and P($-3$): correct. Answer: A, C, D.