Which gives black precipitate on reaction with `CS_(2)` followed by addition of `HgCl_(2)`?

Options

A. `(CH_(3))_(2)CNH_(2)`
B. `(C_(2)H_(5))_(2)NH`
C. `(CH_(3))_(3)N`
D. all the three

Explanation

To solve the question about which compound gives a black precipitate upon reaction with carbon disulfide (CS₂) followed by the addition of mercuric chloride (HgCl₂), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**: The question involves carbon disulfide (CS₂) and mercuric chloride (HgCl₂). We need to determine which organic compound reacts with these substances to produce a black precipitate.

2. **Understand the Reaction with CS₂**: The primary amines (RNH₂) are known to react with carbon disulfide (CS₂). In this reaction, the amine attacks the CS₂, leading to the formation of a dithiocarbamate intermediate.

- **Reaction**:
\[ RNH_2 + CS_2 \rightarrow R-N=C=S \]
- This intermediate can further react, but we will focus on the next step involving HgCl₂.

3. **Reaction with HgCl₂**: The dithiocarbamate formed from the primary amine reacts with mercuric chloride (HgCl₂). This reaction leads to the formation of mercury sulfide (HgS), which is known to be a black precipitate.

- **Reaction**:
\[ R-N=C=S + HgCl_2 \rightarrow HgS (black \ precipitate) + HCl \]
- The black precipitate of mercury sulfide (HgS) is the key product of this reaction.

4. **Conclusion**: The compound that gives a black precipitate when reacted with CS₂ followed by HgCl₂ is primary amines (RNH₂). Therefore, the answer to the question is that primary amines react to give the black precipitate of HgS.

### Final Answer:
**Primary amines (RNH₂) give a black precipitate of mercury sulfide (HgS) when reacted with CS₂ followed by HgCl₂.**