Which of the following best demonstrates the use of an indicator?

The correct choice demonstrates the use of an indicator through the specific action of testing pH levels in a solution. Indicators are substances that provide a visible sign, typically through color changes, in response to a change in conditions, such as acidity or alkalinity. In the context of pH testing, a common indicator might change color at certain pH levels, providing an immediate visual representation of whether the solution is acidic, neutral, or basic. This application aligns perfectly with the purpose of indicators, which is to show the presence or concentration of a specific property—in this case, the acidity level of a solution.

In contrast, identifying a single organism does not necessarily involve an indicator in the same context, as it focuses on classification rather than providing a measurement or sign of a property. Showing involvement in a case pertains to evidence or participation without involving any indication mechanism. Describing a notable event primarily shares information or narratives rather than using an explicit indicator to measure or represent a change or condition.