Correct answer: Seawater

Why?:

The pH is a logarithmic scale with base 10. It is calculated as pH = -log[H⁺]. One common definition of acidity is the ability of a substance to donate H⁺ ions to solution. So the more acidic the substance, the greater H⁺ concentration it produces, and the lower its pH (note the negative sign in the equation). Since the equation that defines pH is logarithmic, a factor of 100 in [H⁺] causes a change in pH level by 2. This means that an acid that is 100 times more acidic than milk of magnesia will have a pH that is 2 lower than the pH of milk of magnesia.

Answering this question:

Locate in the graph milk of magnesia and determine its pH. According to the graph, the pH of milk of magnesia is 10. Anything more acidic than milk of magnesia will have a pH that is lower than 10. Since we are looking for the mixture that is 100 times more acidic than pH 10, we will choose the mixture that has a pH that is lower than 10 by 2. The mixture we are looking for must have a pH of 8 since it is 2 pH values lower than 10. The mixture with a pH of 8 is seawater.

Therefore, seawater is the mixture that is 100 times more acidic than milk of magnesia.