What defines an acid in chemistry?

An acid in chemistry is defined as a substance that increases the concentration of hydrogen ions (H⁺) in a solution. When an acid dissolves in water, it dissociates to release hydrogen ions into the solution, which leads to a decrease in pH and an increase in acidity. This process aligns with the Brønsted-Lowry theory, which identifies acids as proton donors, thereby emphasizing their role in increasing hydrogen ion concentration.

In contrast, other concepts mentioned in the question pertain to different chemical definitions or characteristics. For instance, the notion of a compound that decreases hydrogen ions relates more to bases, which accept protons and thereby can lead to a decrease in hydrogen ion concentration. Neutralization refers to the reaction between an acid and a base that produces water and a salt, which does not encapsulate the core definition of an acid. Lastly, a high pH indicates a basic or alkaline solution, directly opposing the definition of acids, which are characterized by low pH values. Thus, B stands out as the correct definition of an acid due to its direct association with hydrogen ion production.