Which statement best describes enzymes

The answer is D, they influence the rate of chemical reactions. 

Enzymes are made of proteins that can act as biological catalysts, which means it can speed up the chemical reaction under suitable temperatures and pH values and can be reused over and over again.

however, if the temperature or the pH value of the surrounding is out of the optimum range, which is different for different types of enzymes, they may slow down the chemical reactions or even denature and no longer work due to the change of shape in the active site. 

therefore, it can influence the rate of chemical reactions.