Solution a is the most acidic.
In a pH scale of 1 to 14, the smaller the pH is, the more acidic is. In opposite, the higher the pH is, the more alkaline it is. We call substances with pH below 7 as acid, and above 7 as alkali, while 7 as neutral.
Therefore, comparing the 4 solutions, solution a has the lowest pH, 3, so it is the most acidic.
