Hydrogen ion concentration in a solution is called pH so, option 'D' is the correct answer.
What is pH ?
The concentration of hydrogen ions in a solution is measured by its pH, which is a measure of its acidity. Pure water splits into hydrogen and hydroxyl (OH-) ions in roughly equal amounts.
The word pH denotes the concentration of hydrogen ions in a solution, such as what percentage of a solution contains hydrogen ions. The pH scale ranges from 1 to 14, with 1 being the most acidic and 14 being the most alkaline.
Find out more information about 'alkaline'.
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