Answer : Option D) Decreases by a factor of 1000.
Explanation : When the pH of lower concentration of hydronium ion [tex] H^{+} [/tex] is increased the pH values decreases by the factor of 1000.
From, the relation of pH and the concentration of hydronium ion we can relate it easily;
pH = - log [[tex] H^{+} [/tex] ]
So, when the pH is lower the concentration of hydronium ion will increase in the solution and vice-versa.