The correct answer is option 4: carbon-dioxide from the air is dissolved.

When water is aerated, it comes into contact with the air, which contains various gases, including carbon dioxide (CO2). Carbon dioxide dissolves in water to form carbonic acid (H2CO3). This reaction occurs because water can act as a weak acid and undergoes a process called hydration, where it combines with CO2 to form carbonic acid.

Carbonic acid is a weak acid, so when it forms in water, it increases the concentration of hydrogen ions (H+) in the solution, making the water slightly acidic. The increased concentration of hydrogen ions lowers the pH of the water, causing it to become slightly acidic.

It is important to note that pure water does not contain carbonic acid, and it has a pH of 7, which is considered neutral. However, when exposed to air and carbon dioxide, it undergoes a chemical reaction, leading to the formation of carbonic acid and a decrease in pH value, making the water slightly acidic.