The answer is CH3COONa.
This is due to common ion effect i.e, when a strong electrolyte suppresses the dissociation power of the weaker electrolyte.

The concentration of H+ ions in a solution can be affected by different factors, including the presence of certain substances. Let's analyze each of the options provided to determine if they would decrease the H+ ion concentration of an acetic acid solution.

A. AgNO3 (Silver Nitrate):
- When AgNO3 is dissolved in water, it dissociates into Ag+ and NO3- ions.
- The presence of Ag+ ions does not directly affect the H+ ion concentration of the acetic acid solution.
- Therefore, AgNO3 would not decrease the H+ ion concentration of the solution.

B. CH3COONa (Sodium Acetate):
- When CH3COONa is dissolved in water, it dissociates into Na+ and CH3COO- ions.
- The CH3COO- ions can react with H+ ions, forming acetic acid (CH3COOH) molecules.
- This reaction decreases the concentration of H+ ions in the solution.
- Therefore, CH3COONa would decrease the H+ ion concentration of the acetic acid solution.

C. Al2(SO4)3 (Aluminum Sulfate):
- When Al2(SO4)3 is dissolved in water, it dissociates into Al3+ and SO4^2- ions.
- The presence of Al3+ ions does not directly affect the H+ ion concentration of the acetic acid solution.
- Therefore, Al2(SO4)3 would not decrease the H+ ion concentration of the solution.

D. NH4Cl (Ammonium Chloride):
- When NH4Cl is dissolved in water, it dissociates into NH4+ and Cl- ions.
- The NH4+ ions can react with water, forming NH3 (ammonia) and H3O+ (hydronium) ions.
- This reaction decreases the concentration of H+ ions in the solution.
- Therefore, NH4Cl would decrease the H+ ion concentration of the acetic acid solution.

In summary, out of the given options, CH3COONa and NH4Cl would decrease the H+ ion concentration of an acetic acid solution.