Step 1: Understand enzymes.
Enzymes are biological catalysts made by living cells. Like all catalysts, they speed up a reaction but are not used up in it.
Step 2: Recall activation energy.
Every reaction needs a minimum amount of energy to get going, called the activation energy. If this is high, the reaction is slow.
Step 3: See how enzymes help.
An enzyme offers an easier path with lower activation energy. This lets more molecules react successfully, so the reaction goes faster.
Step 4: Rule out wrong options.
Enzymes do not raise the temperature of the reaction, and they do not stop or block the reaction. Those choices are wrong.
Step 5: Note the change in mechanism.
While an enzyme does provide a new pathway, its main and most important job is simply to make the reaction happen quicker.
Step 6: Conclude.
So the chief role of enzymes is to increase the reaction rate.
\[ \boxed{\text{Increase reaction rate}} \]