Final answer:
CH₃CO₂H behaves as an acid, while Mg(OH)₂, NaOH, and HI behave as bases when dissolved in water.
Step-by-step explanation:
When a compound dissolves in water, it can either behave as an acid, a base, or remain neutral. To determine which compounds behave as acids, we need to look for compounds that release hydrogen ions (H+) when dissolved in water. The compound CH₃CO₂H, also known as acetic acid, is an example of an acid. It ionizes in water to form acetate ions (CH₃COO-) and hydrogen ions (H+). Therefore, option a) CH₃CO₂H behaves as an acid. The other compounds, Mg(OH)₂, NaOH, and HI, all behave as bases when dissolved in water.