`KCl` is used in salt bridge because:

Options

A. It forms a good jelly with agar-agar
B. It is a strong electrolyte
C. It is a good conductor of electricity
D. Migration factor of `K^(+) and Cl^(-)` ions are almost equal

Answer

### Step-by-Step Solution:

1. **Understanding the Purpose of a Salt Bridge**:
- A salt bridge is an essential component in an electrochemical cell. Its primary function is to maintain electrical neutrality in the half-cells by allowing the flow of ions.

2. **Identifying the Properties of KCl**:
- KCl (potassium chloride) is a strong electrolyte, which means it dissociates completely in solution into K⁺ and Cl⁻ ions. This property is crucial for the conduction of electricity in the salt bridge.

3. **Analyzing the Migration Factors**:
- The migration factor of K⁺ and Cl⁻ ions should be nearly equal. This ensures that the movement of these ions does not create a buildup of charge in either half-cell, which would disrupt the electrochemical reaction.

4. **Considering Ionic Mobility**:
- The ionic mobility of K⁺ and Cl⁻ ions is comparable, meaning they can move through the solution at similar rates. This is important for maintaining the balance of charge as the electrochemical reaction proceeds.

5. **Conclusion**:
- KCl is used in salt bridges because it meets the necessary criteria: it is a strong electrolyte, it forms a good jelly with agar (which can help in the physical structure of the bridge), it conducts electricity well, and the migration factors of its ions are nearly equal. This ensures effective functioning of the salt bridge in maintaining charge neutrality and completing the circuit in an electrochemical cell.