The ph scale measures
a. the strength of an acid.
b. the strength of hydroxide ions.
c. the concentration of hydrogen ions.
d. the concentration of an base.

C. the concentration of Hydrogen ions. 

pH is concentration of H ions while pOH is the concentration of hydronium ions

c. the concentration of hydrogen ions.

Reaction rate is affected by concentration. When the concentration of the reactants are raised, the rate of reaction is increased as well. Increase in concentration promotes more effective collisions. The exact relation of reaction rates on concentration is dependent on the mechanism of the reaction.