Which of the following solutions is considered more acidic?

Lemon juice is considered more acidic primarily because of its high concentration of citric acid, which gives it a low pH value typically around 2 to 3. This means that lemon juice contains a higher concentration of hydrogen ions compared to the other options listed, thus making it more acidic. In comparison, milk of magnesia, which is a basic solution due to the presence of magnesium hydroxide, has a pH greater than 7. Water generally has a neutral pH of 7, and tap water may slightly vary but is also close to neutral, generally ranging between 6.5 and 8.5.

The characteristic of acidity is marked by the ability of a solution to donate protons (H⁺ ions), and the more prevalent these ions, the lower the pH, making the solution acidic. Therefore, lemon juice stands out distinctly among the choices as having the strongest acidic nature.