The correct answer is A. Increase.
When the CO2 concentration in the blood increases, the breathing will increase. This is because the body needs to get rid of the excess CO2, which is a waste product of metabolism. The respiratory center in the brain detects the increase in CO2 and sends signals to the muscles that control breathing, causing them to contract more forcefully. This increases the rate and depth of breathing, which helps to remove the excess CO2 from the blood.
Option B is incorrect because the breathing will increase, not decrease. Option C is incorrect because the breathing will not stop. Option D is incorrect because the breathing will have an effect on the body.