Concept:An indicator is a substance whose colour changes with the pH, allowing us to identify whether a solution is acidic or basic.
Explanation:Phenolphthalein is the common laboratory indicator used to test for bases.
In acidic and neutral solutions, phenolphthalein remains colourless.
Once the solution becomes basic, that is
pH>8.3, it turns pink or purple.
This distinct colour change is easy to observe, so a few drops are usually added to the test solution.
When the solution turns pink, it confirms that the solution contains a base.
It is also widely used in titration experiments to mark the endpoint when a base is involved.
Methyl orange is better for acids, while litmus and bromothymol blue are general indicators, but phenolphthalein gives the clearest result for a base.
Answer:B. Phenolphthalein