Compound that will give positive Lassaigne's test for both nitrogen and halogen is

Lassaigne's test detects nitrogen and halogens in organic compounds by fusing the compound with sodium metal. The key reactions are:

For nitrogen: $$\text{Na} + \text{C} + \text{N} \rightarrow \text{NaCN}$$

For halogens: $$\text{Na} + \text{X} \rightarrow \text{NaX}$$

For the test to detect nitrogen, the compound must contain carbon (to form NaCN).

Let us examine each option:

Option A: $$\text{N}_2\text{H}_4 \cdot \text{HCl}$$ (Hydrazine hydrochloride) — Contains N and Cl, but no carbon. Without carbon, NaCN cannot form, so nitrogen detection fails.

Option B: $$\text{CH}_3\text{NH}_2 \cdot \text{HCl}$$ (Methylamine hydrochloride) — Contains C, N, and Cl. Carbon is present, so NaCN forms (positive nitrogen test). Cl is present, so NaCl forms (positive halogen test). Both tests are positive.

Option C: $$\text{NH}_4\text{Cl}$$ — Contains N and Cl, but no carbon. Nitrogen test fails.

Option D: $$\text{NH}_2\text{OH} \cdot \text{HCl}$$ (Hydroxylamine hydrochloride) — Contains N and Cl, but no carbon. Nitrogen test fails.

The answer is Option B: $$\text{CH}_3\text{NH}_2 \cdot \text{HCl}$$.