Which of the following is a common indicator of alkalinity in a solution?

Phenolphthalein is widely recognized as a common indicator of alkalinity in a solution. It has a distinct property where it changes color depending on the pH level of the solution. Specifically, phenolphthalein is colorless in acidic solutions (pH below 7) and turns pink in neutral to basic (alkaline) conditions (pH around 8.2 and above). This clear transition makes it an effective choice for identifying alkaline solutions.

In contrast, while litmus, methyl red, and universal indicators also serve as pH indicators, they do not specifically highlight the point at which a solution becomes alkaline as effectively as phenolphthalein. Litmus paper shows red in acidic solutions and blue in basic solutions, but it does not provide a precise measurement of pH. Methyl red turns red at a pH below 4.4 and yellow above 6.2, which focuses more on acidic to neutral ranges. Universal indicators can indicate a range of pH values but may not provide the clear transition that phenolphthalein offers specifically for alkaline solutions.