Which of the following is a characteristic of acids?

Acids are known for their characteristic sour taste, which is often associated with many common acidic substances such as lemon juice and vinegar. This sour flavor comes from the presence of hydrogen ions (H⁺) in the solution, which stimulate the sour taste receptors on our taste buds.

The other characteristics listed are not typical of acids. For instance, bitter taste is usually a characteristic of bases, not acids. Additionally, turning red litmus paper blue signifies the presence of a base, which is the opposite behavior of acids, which turn blue litmus paper red. Bases also tend to produce a slippery feel when dissolved in water, which is another distinct property that differentiates them from acids. Therefore, the correct answer highlights a fundamental trait of acids that is easily observable in daily life and important for understanding acid-base chemistry.