Which solution among the given options has the lowest pH value and therefore the greatest number of hydrogen ions?

The solution with the lowest pH value indicates a higher concentration of hydrogen ions (H⁺) in that solution. pH is a logarithmic scale, where each whole number change on the scale represents a tenfold change in acidity or hydrogen ion concentration. This means that as the pH value decreases, the concentration of hydrogen ions increases.

In this case, the solution with a pH of 2.5 has the lowest pH compared to the other solutions listed. A pH of 2.5 is significantly more acidic than the others, meaning it has a greater number of hydrogen ions present in that solution. A pH of 3.7 is still acidic, but not as much as 2.5. pH values like 7.1 are considered neutral (pure water), and a pH of 9.0 is basic, indicating a lower concentration of H⁺ ions.

Thus, since a lower pH corresponds directly to a higher hydrogen ion concentration, the solution with a pH of 2.5 is indeed the correct choice for having the greatest number of hydrogen ions.