The equilibrium
`Al(OH)_3+OH^(-)hArr [Al(OH)_4]^(-)`
in aqueous medium shifts towards the left in the presence of :

Options

A. `NH_4Cl`
B. Dilute `H_2SO_4` (not in excess)
C. NaOH
D. carbon dioxide gas

Explanation

To determine which reagent causes the equilibrium reaction

\[ \text{Al(OH)}_3 + \text{OH}^- \rightleftharpoons \text{[Al(OH)}_4]^-\]

to shift towards the left in an aqueous medium, we need to analyze how each reagent affects the concentration of hydroxide ions (OH⁻) and the aluminum hydroxide complex.

### Step-by-Step Solution:

1. **Understanding the Equilibrium**:
The equilibrium involves aluminum hydroxide (Al(OH)₃) and its complex ion (Al(OH)₄⁻). The left side represents the formation of Al(OH)₃, while the right side represents the formation of Al(OH)₄⁻.

2. **Effect of Reagents**:
- **NH₄Cl**: When NH₄Cl is added, NH₄⁺ ions can react with OH⁻ ions, reducing their concentration. This decrease in OH⁻ concentration shifts the equilibrium to the left, favoring the formation of Al(OH)₃ precipitate.
- **Dilute H₂SO₄**: The addition of dilute H₂SO₄ introduces H⁺ ions, which also react with OH⁻ ions, further decreasing their concentration. This shift in equilibrium also favors the formation of Al(OH)₃ precipitate.
- **Sodium Hydroxide (NaOH)**: NaOH increases the concentration of OH⁻ ions, which would shift the equilibrium to the right, favoring the formation of Al(OH)₄⁻. Therefore, it does not cause a shift to the left.
- **Carbon Dioxide (CO₂)**: When CO₂ is introduced, it can react with the hydroxide ions and form bicarbonate (HCO₃⁻), effectively reducing the concentration of OH⁻ ions. This also shifts the equilibrium to the left, favoring the formation of Al(OH)₃ precipitate.

3. **Conclusion**:
The equilibrium shifts towards the left in the presence of NH₄Cl, dilute H₂SO₄, and carbon dioxide (CO₂). Therefore, the correct answer includes these three reagents.

### Final Answer:
The equilibrium shifts towards the left in the presence of:
- NH₄Cl
- Dilute H₂SO₄
- Carbon Dioxide (CO₂)