The characteristics of an acid include:
- a) donates protons: Acids are defined as substances that can donate protons (H⁺) according to the Bronsted-Lowry theory.
- c) tastes sour: Many acids have a sour taste, which is a common characteristic of acidic substances.
The correct answers among the options provided are a and c. However, since you asked for a single characteristic, a) donates protons is the most definitive characteristic of an acid.