Which of the following is not an indicator that occurs naturally?

Phenolphthalein

The correct answer is option 4. Phenolphthalein.

Phenolphthalein is not an indicator that occurs naturally. It is a synthetic chemical compound commonly used as a pH indicator in laboratory and educational settings. The other options mentioned—Gudhal (Hibiscus), Turmeric, and Litmus—are natural indicators.

Therefore, the correct option is Phenolphthalein.