Indicators exhibit different colours in acidic and basic solutions.   hence correct answer is A.

Introduction:
An indicator is a substance that undergoes a noticeable change when conditions such as pH, temperature, or concentration are altered. It is commonly used to determine the endpoint of a chemical reaction or to monitor changes in a system. In this case, we are considering the usefulness of an indicator based on its color change.

Explanation:
The color change of an indicator is the most important factor in determining its usefulness. This is because the color change provides a visible and easily recognizable signal that a change has occurred in the system being monitored. Let's explore the reasons why the color change of an indicator is crucial.

1. Visual indication:
The primary purpose of an indicator is to provide a visual indication of a change. A color change is easily detectable by the human eye, making it an ideal way to observe and monitor changes in a system.

2. Qualitative analysis:
Color changes in indicators are often used in qualitative analysis to identify the presence or absence of certain substances. By observing the color change, one can determine the pH, presence of ions, or other chemical properties of a solution.

3. Endpoint determination:
In chemical reactions, indicators are commonly used to determine the endpoint, which is the point at which the reaction is considered complete. For example, in a titration, the color change of an indicator signals that the reaction between the analyte and titrant is complete, allowing for accurate measurement of the quantity of the analyte.

4. Ease of use:
Color changes are relatively easy to observe and interpret, making indicators user-friendly. They can be used in various settings, including laboratories, classrooms, and even in everyday life.

5. Wide range of indicators:
There are numerous indicators available, each with a specific color change range. This allows for versatility in choosing an indicator that is suitable for a particular application or system.

Conclusion:
In conclusion, the usefulness of an indicator depends on its ability to undergo a color change. The color change serves as a visual indication, aids in qualitative analysis, helps determine reaction endpoints, is easy to observe and interpret, and offers a wide range of options. Therefore, option 'A' - its color, is the correct answer in this case.