What does pH measure in a solution?

The correct answer highlights the concept that pH is a measure of hydrogen ion concentration, which reflects the balance between acidity and basicity in a solution. Specifically, pH assesses the negative logarithm of the hydrogen ion concentration, providing a numerical value that indicates whether a solution is acidic, neutral, or basic.

When pH is mentioned in the context of equal acid and base ions, it relates to how a neutral solution (with a pH of 7) has equal concentrations of hydrogen ions and hydroxide ions. This balance is essential for determining the chemical nature of the solution. Therefore, stating that acid and base ions are equal ties directly to the concept of pH representing the balance between these ions.

While other options may seem relevant, they do not fully encapsulate the primary function of pH as accurately as the chosen answer does. For example, acidity alone pertains only to the concentration of hydrogen ions without considering their balance with hydroxide ions. Similarly, the total number of ions does not specifically convey information about the acidity or basicity of the solution, and the concentration of hydroxide ions alone does not define pH since it is dependent on hydrogen ion concentration to derive a pH value.