The correct answer is a. Measures hydroxide ion concentration (strength of the acid or base) because when you look at the scientific definition of the use of a PH scale it says... "Acidity is the amount of hydrogen protons present in a solution and basicity is the amount of hydroxide ions present in a solution. The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A pH of 7 is neutral."
