To increase significantly the concentration of free `Zn^(2+)` ion is a solution of the complex ion `[Zn(NH_(3))_(4)]^(2+)`
`Zn^(2+) (aq)+4NH_(3) (aq) hArr [Zn(NH_(3))_(4)]^(2+) (aq)`
Add to the solution some:

Options

A. `H_(2)O`
B. `HCl(aq)`
C. `NH_(3)(aq)`
D. `NH_(4)Cl(aq)`

Answer

To solve the problem of how to increase the concentration of free \( \text{Zn}^{2+} \) ions in a solution of the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \), we need to analyze the equilibrium reaction given:

\[ \text{Zn}^{2+} (aq) + 4 \text{NH}_3 (aq) \rightleftharpoons [\text{Zn(NH}_3)_4]^{2+} (aq) \]

### Step-by-Step Solution:

1. **Understanding the Equilibrium**: The reaction shows that \( \text{Zn}^{2+} \) ions combine with ammonia (\( \text{NH}_3 \)) to form the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \). To increase the concentration of free \( \text{Zn}^{2+} \) ions, we need to shift the equilibrium to the left.

2. **Le Chatelier's Principle**: According to Le Chatelier's Principle, if we change the conditions of a system at equilibrium, the system will adjust to counteract that change. In this case, if we decrease the concentration of \( \text{NH}_3 \), the equilibrium will shift to the left to produce more \( \text{NH}_3 \) and, consequently, more \( \text{Zn}^{2+} \) ions.

3. **Adding HCl**: When we add hydrochloric acid (HCl) to the solution, it reacts with ammonia (\( \text{NH}_3 \)) to form ammonium chloride (\( \text{NH}_4\text{Cl} \)). This reaction reduces the concentration of \( \text{NH}_3 \) in the solution.

\[ \text{NH}_3 (aq) + \text{HCl} (aq) \rightarrow \text{NH}_4\text{Cl} (aq) \]

4. **Effect on the Equilibrium**: As the concentration of \( \text{NH}_3 \) decreases due to the formation of \( \text{NH}_4\text{Cl} \), the equilibrium will shift to the left to compensate for the loss of \( \text{NH}_3 \). This shift will result in the decomposition of the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \), thereby increasing the concentration of free \( \text{Zn}^{2+} \) ions in the solution.

5. **Conclusion**: Therefore, the correct choice to increase the concentration of free \( \text{Zn}^{2+} \) ions in the solution is to add **HCl**.

### Final Answer:
Add **HCl** to the solution.