Answer : The correct option is, pH meter
Explanation :
pH meter : It is a type of instrument which measures the activity of hydrogen ion in water based solution. Or we can say that it will provide the most direct measure of pH by measuring the hydrogen ion concentration of a solution.
Litmus paper : It is a type of paper which changes the color based on whether the solution is an acid or a base.
Phenolphthalein : It is an indicator which changes the color based on pH values.
Bromthymol blue : It does not tell the concentrations, it is a pH indicator for the weak acids and bases.
Hence, the pH meter provide the most direct measure of pH by measuring the hydrogen ion concentration of a solution.