Final answer:
A solution with a pH of 3 is 10 times more acidic than a solution with a pH of 4 because the pH scale is logarithmic and each unit change represents a tenfold change in hydrogen ion concentration.
Step-by-step explanation:
The pH scale is logarithmic, which means that each whole number change on the pH scale corresponds to a tenfold change in the concentration of hydrogen ions ([H+]). Therefore, a solution with a pH of 3 is 10 times more acidic than a solution with a pH of 4, because the pH is one unit higher. Conversely, a solution with a pH of 2 would be 10 times more acidic than a solution with a pH of 3. The correct answer to the student's question is (c) 4, as it is the pH value that is one unit higher than 3.