Answer: Option (a) is the correct answer.
Step-by-step explanation:
A substance that gives a visible change in color upon exposure to certain conditions is known as an indicator. Indicators are either weak acids or bases.
For example, phenolphthalein is an acid-base indicator which changes its color from colorless to pink in a basic solution at pH 8.2.
In a more basic solution, its color changes to bright purple.
Hence, indicators do not maintain their colors across the range of pH values for which they are used.
A pH meter tells exactly the pH value of a solution or substance. Whereas an indicator only tells whether a solution is acidic or basic.
Therefore, we can conclude that it is true about indicators that they are weak acids or bases.