Final answer:
The pH 14 has the highest concentration of hydroxide ions.
Step-by-step explanation:
The concentration of hydroxide ions can be determined using the pH scale. The pH scale ranges from 0 to 14, with values below 7 indicating an acidic solution, values above 7 indicating a basic solution, and a pH of 7 indicating a neutral solution. In this case, the pH 14 has the highest concentration of hydroxide ions, as it represents a very basic solution.