Which of the following characteristics is consistent with an acid?

The characteristic that best describes an acid is that it reduces pH to below 7. Acids, by definition, are substances that can donate protons (H+) to other substances. This proton donation lowers the pH of a solution, resulting in a more acidic environment.

When discussing the other characteristics mentioned, while acids can react with metals like zinc to produce hydrogen gas—this is indeed a property of certain acids—it's not a defining characteristic of all acids. Many acids do not react with zinc, so this option does not universally apply to all acids.

Similarly, while acids can neutralize bases and react with them to form salts, these characteristics describe their behavior in chemical reactions rather than defining what an acid is. The primary characteristic that consistently indicates an acid is its ability to lower the pH of a solution below 7. This emphasizes the central role of pH in identifying acidic substances.