The Lassaigne’s extract is first boiled with dil. HNO₃ to decompose cyanide or sulphide of sodium (NaCN or Na₂S) formed during Lassaigne’s test. These ions would otherwise interfere with silver nitrate test for halogen.

$\mathrm{NaCN} ( \mathrm{aq} ) + {\mathrm{HNO}}_3 ( \mathrm{aq} ) \to {\mathrm{NaNO}}_3 ( \mathrm{aq} ) + \mathrm{HCN} (\uparrow )$

${\mathrm{Na}}_2\mathrm{S} ( \mathrm{aq} ) + 2 {\mathrm{HNO}}_3 ( \mathrm{aq} ) \to 2 {\mathrm{NaNO}}_3 ( \mathrm{aq} ) + {\mathrm{H}}_2\mathrm{S} ( \uparrow )$

Hence option D is the answer.