`HgCl_2` is a covalent compound, sparingly soluble in water, the solubility increase by the addition of chloride ions due to:

Options

A. common ion effect
B. formation of complex `[HgCl_(4)]^(2-)`
C. weakening of Hg-Cl bonds
D. strong ion-dipole forces

Answer

To solve the question regarding the solubility of \( \text{HgCl}_2 \) in the presence of chloride ions, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Compound**:
- \( \text{HgCl}_2 \) (mercuric chloride) is a covalent compound that is sparingly soluble in water. This means that it does not dissolve well in water under normal conditions.

2. **Effect of Adding Chloride Ions**:
- When chloride ions (\( \text{Cl}^- \)) are added to a solution containing \( \text{HgCl}_2 \), the solubility of \( \text{HgCl}_2 \) increases.

3. **Formation of a Complex Ion**:
- The addition of chloride ions leads to the formation of a complex ion. Specifically, the reaction can be represented as:
\[ \text{HgCl}_2 + 2\text{Cl}^- \rightarrow \text{HgCl}_4^{2-} \]
- In this reaction, \( \text{HgCl}_2 \) reacts with two chloride ions to form the complex ion \( \text{HgCl}_4^{2-} \).

4. **Reason for Increased Solubility**:
- The formation of the complex ion \( \text{HgCl}_4^{2-} \) effectively removes \( \text{HgCl}_2 \) from the solution as it shifts the equilibrium. This results in more \( \text{HgCl}_2 \) dissolving to maintain the equilibrium, thereby increasing the overall solubility of \( \text{HgCl}_2 \).

5. **Conclusion**:
- The increase in solubility of \( \text{HgCl}_2 \) upon the addition of chloride ions is primarily due to the formation of the complex ion \( \text{HgCl}_4^{2-} \).

### Final Answer:
The solubility of \( \text{HgCl}_2 \) increases with the addition of chloride ions due to the formation of the complex ion \( \text{HgCl}_4^{2-} \).