Final answer:
As the activation energy increases, the reaction rate tends to slow down due to the requirement of more energy for molecules to collide successfully and react. Hence the correct answer is option D- The reaction begins to slow down.
Step-by-step explanation:
When the activation energy required for a chemical reaction to proceed increases, the rate at which the reaction occurs tends to slow down. This is because higher activation energy means that the molecules participating in the reaction need to possess greater energy to successfully collide and react. The source of activation energy is typically heat energy from the surroundings, which increases the motion and collision frequency of molecules. However, if the activation energy barrier is too high, there may not be enough kinetic energy in the system to overcome this barrier, leading to fewer successful collisions and a slower reaction rate.