Question

Which factor decreases the rate of a chemical reaction within a cell?
A co-factor
a increased enzyme
an increased substrate
cold temperatures

Answer 1

the second one
an increased enzyme

Answer 2

Adding a co-factor may increase the reaction rate if it's part of the enzyme that catalyzes the reaction. Otherwise, adding a co-factor probably won't have an effect.

Increasing the amount of enzyme would increase the reaction rate because you're adding more catalyst to the system.

Increasing the amount of substrate would also increase the reaction rate because reaction rate is (often) proportional to substrate concentration.

That leaves D as your answer. Cold temperatures mean reactants in the system have less kinetic energy, meaning they are less likely to collide and undergo the reaction.