The correct way to express the concentration of a solution is parts per billion. Option A
What is concentration?
Concentration in parts per billion (ppb) is a unit of measurement used to express the amount of one substance (the solute) in relation to a billion (10^9) parts of another substance (the solution). It is a way to represent very small concentrations in a ratio of parts to a billion parts.
Parts per billion is commonly used in environmental science, chemistry, and toxicology to express trace concentrations of pollutants, chemicals, or other substances in air, water, soil, or biological samples.