What unit of measurement is commonly used to indicate pH levels?

The commonly used unit of measurement to indicate pH levels is the pH scale itself, which is dimensionless rather than associated with a specific unit like liters, millimeters, or grams. The pH scale ranges typically from 0 to 14, with 7 being neutral, values below 7 indicating acidity, and values above 7 indicating alkalinity. This scale is a logarithmic measure of hydrogen ion concentration in a solution, rather than a physical measurement requiring a unit of measurement. Therefore, stating "None of the above" correctly identifies that pH does not use traditional units like liters, millimeters, or grams to express its levels.