__________ is an acid-base indicator and turns red when acid is added to it.

The correct answer is: Red cabbage

Anthocyanins are a large group of molecules that all have the same basic structure, containing three rings of atoms. Each anthocyanin contains the same core structure which is crucial for absorbing and reflecting light to make a color. This structure happens to be very sensitive to the acidity of the environment surrounding it. It influences how it organizes the electrons within the molecule.
Red cabbage has anthocyanin. The type of anthocyanins present determines just how they will respond to a change in the pH-value of the environment. In the case of red cabbage the color roughly changes as follows

• pH ≈ 2: red/bright pink, lime juice (which we sprinkled on top of our salad) will cause the pH to sink down this low and make the cabbage red
• pH ≈ 4: light purple
• pH ≈ 6: violet, this is approximately the color of ‘native’ red cabbage
• pH ≈ 8: blue
• At an even higher pH value, the red cabbage can even turn green/yellowish.