An acid base indicator is a dye that is:

a. Always the same color

b. Colorless

c. The same color in both acids and bases

d. One color in an acid and a different color in a base

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.