Which will provide the most direct measure of pH by measuring the hydrogen ion concentration of a solution?

A. pH meter

B. litmus paper

C. phenolphthalein

D. bromthymol blue

A. pH meter is the instrument which will provide the most direct measure of pH by measuring the hydrogen ion concentration of a solution.

Litmus paper is just a paper which changes color based on whether the solution is an acid or a base. Phenolphthalein is an indicator which changes color based on pH values. And bromthymol blue doesn't tell concentrations, it is just a pH indicator for weak acids and bases.