Which pH change represents a hundredfold increase in the concentration of H3O+?

(1) pH 5 to pH 7 (3) pH 3 to pH 1 (2) pH 13 to pH 14 (4) pH 4 to pH 3

The answer is (3) pH 3 to pH 1. The pH is related to the concentration of H3O + with the relationship: pH = - lg c (H+). So when concentration of H3O + increase, the pH will decrease. And decrease 2 when when increase hundredfold. Because 100=10^2.