You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down.

what can you do to speed the reaction up again?

Add more inhibitor to speed up the reaction.

Add more substrate; it will outcompete the inhibitor and increase the reaction rate.

Increase the temperature.

Increase the pH.

Add more substrate.

Explanation:

First let's explain why all the other options are wrong:

The problem states that we have optimum pH and optimum temperature, this means that any change to those factors (increasing or decreasing) would slow down the reaction. An inhibitor is called that way because its presence slows downs the reaction, so more inhibitor would not speed up the reaction.

A competitive inhibitor is a substance that resembles the substrate that normally reacts with the enzime, so it binds to the enzime's active sites and prevents the substrate from doing so. Adding more substrate makes it more likely than a substrate molecule binds to an enzime's active site instead of the inhibitor ("it will outcompete the inhibitor and increase the reaction rate").