Which of the following is an acid-base indicator? 

Turmeric 

The correct answer is option 3. Turmeric.

Turmeric is a natural substance commonly used as a spice in cooking, and it contains a compound called curcumin. Curcumin has the ability to exhibit different colors in solutions with varying pH levels, making turmeric a useful acid-base indicator.

Here is how turmeric functions as an acid-base indicator:

1. Acidic Conditions: In acidic solutions, turmeric appears yellow. The color change is due to the interaction between curcumin and the hydrogen ions (H⁺) present in the acidic solution.

2. Neutral to Alkaline Conditions: In neutral to alkaline solutions, turmeric turns reddish-brown. This change in color occurs when curcumin interacts with hydroxide ions (OH⁻) in alkaline solutions.

Turmeric is often used in traditional and laboratory settings to detect the presence of acids or bases based on the observed color change. It's a natural and easily accessible indicator that undergoes a visible transformation, making it a practical tool for simple pH testing.

While turmeric is a reliable indicator for certain pH ranges, it may not be as precise as some synthetic indicators in laboratory settings. Nonetheless, it is a versatile and cost-effective option for educational and household purposes.