What changes color in the presence of an acid or base?

The correct choice is an indicator because indicators are substances that change color in response to acidity or basicity of a solution. They are commonly used in titrations and pH tests to visually show the pH level of a solution. The presence of an acid or base alters the structure of the indicator, leading to a visible change in its color, allowing for easy identification of the pH range.

In contrast, a catalyst speeds up a chemical reaction without being consumed but does not exhibit color changes related to acidity or basicity. A buffer helps maintain a stable pH when acids or bases are added, but it does not change color itself. An electrolyte is a substance that produces an electric current when dissolved in water but, again, does not change color upon interacting with acidic or basic substances. Thus, an indicator is the essential tool for detecting changes in pH through color change.