Answer: The correct answer is Option d.
Explanation:
An indicator is defined as the substance which shows the presence of acid or a base in a solution. They change their color accordingly.
Acid-base indicators are usually weak acids or weak bases. They change their color in the solution with change in the pH.
For Example: Phenolphthalein is an acid-base indicator which remains colorless in acidic solution and changes its color to pink when added to basic solution.
Hence, the correct answer is Option d.
