In Lassaigne's test, the following reagent is used to test the presence of both N and S.

Options

A. `AgNO_(3)`
B. `FeCl_(3)`
C. `Na_(2)S`
D. `(CH_(3)COO)_(2)Pb`.

Explanation

### Step-by-Step Solution:

1. **Understanding Lassaigne's Test**: Lassaigne's test is a qualitative analysis method used to detect the presence of certain elements (like nitrogen and sulfur) in organic compounds. It involves the formation of specific ionic species when the organic compound is heated with sodium.

2. **Reaction with Sodium**: When the organic compound containing nitrogen (N) and sulfur (S) is heated with sodium, it forms sodium thiocyanate (NaCNS). The reaction can be represented as:
\[ \text{Organic Compound} + \text{Na} \rightarrow \text{NaCNS} \]

3. **Testing for Nitrogen and Sulfur**: The sodium thiocyanate (NaCNS) formed is then treated with ferric chloride (FeCl3). This reaction helps in identifying the presence of both nitrogen and sulfur:
\[ \text{NaCNS} + \text{FeCl}_3 \rightarrow \text{Fe(CNS)}_3 + \text{NaCl} \]
Here, ferric thiocyanate (Fe(CNS)3) is produced, which is a complex that indicates the presence of both elements.

4. **Observation of Color Change**: The ferric thiocyanate formed is characterized by a blood-red color. The appearance of this color indicates that both nitrogen and sulfur are present in the organic compound.

5. **Conclusion**: Therefore, the reagent used in Lassaigne's test to detect the presence of both nitrogen and sulfur is **sodium (Na)**, which reacts to form sodium thiocyanate, and subsequently, ferric chloride is used to confirm the presence of these elements through the formation of blood-red ferric thiocyanate.

### Final Answer:
The reagent used in Lassaigne's test to test the presence of both nitrogen and sulfur is **sodium (Na)**.

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