Which of the following is a characteristic of a catalyst?

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy needed for the reaction to proceed. By facilitating this process, a catalyst helps reactants convert into products more quickly without being consumed or altered in the overall reaction. This characteristic makes catalysts essential in various chemical processes, including industrial and biological reactions.

In contrast, other characteristics mentioned in the question do not align with the role of a catalyst. For instance, increasing activation energy or changing during the reaction would negate the definition and functionality of a catalyst. Additionally, while a catalyst helps accelerate reactions, it does not stop them. Instead, it ensures that the reaction occurs more efficiently. Thus, the primary role of a catalyst to speed up chemical reactions is the correct and defining characteristic.