Which of the following is a characteristic of acids?

Acids are substances that, when dissolved in water, release hydrogen ions (H⁺), which is a defining characteristic. One of the classic tests for the presence of an acid is its effect on litmus paper, a pH indicator. Acids will turn blue litmus paper red due to the increase in hydrogen ion concentration, which alters the color of the pH indicator in the presence of acidic conditions.

The other characteristics listed are typically associated with bases rather than acids. For instance, bases feel slippery, have a pH greater than 7, and produce hydroxide ions (OH⁻) in water. Thus, the specific ability of acids to turn blue litmus paper red effectively identifies them in a laboratory or educational setting.