The Lassiagne's extract is boiled with dil $\mathrm{HNO}_3$ before testing for halogens because,

1. A Silver halides are soluble in $\mathrm{HNO}_3$.
2. B $\mathrm{AgCN}$ is soluble in $\mathrm{HNO}_3$.
3. C $\mathrm{Na}_2 \mathrm{S}$ and $\mathrm{NaCN}$ are decomposed by $\mathrm{HNO}_3$. Correct answer
4. D $\mathrm{Ag}_2 \mathrm{S}$ is soluble in $\mathrm{HNO}_3$.