Which of the following substances is a common biological catalyst?

A common biological catalyst is an enzyme, which is primarily composed of proteins. Enzymes facilitate biochemical reactions by lowering the activation energy required for the reaction to occur, thereby increasing the reaction rate without being consumed in the process. This catalytic property is fundamental to various biological functions, including digestion, metabolism, and DNA replication.

While DNA plays a crucial role in the storage and transmission of genetic information, it does not act as a catalyst in biological reactions. Water, although vital for life and participating in many biochemical reactions, does not serve as a catalyst itself. Salt, made up of ions, can influence reactions through osmosis or ionic strength but does not catalyze reactions in the way proteins do. Therefore, proteins, specifically enzymes, are recognized for their catalytic abilities in biological systems.