**Core Concept**
The question is testing the understanding of the respiratory response to changes in carbon dioxide (CO2) levels in the blood. Specifically, it's assessing the response to an increase in PCO2, which is a critical aspect of acid-base balance regulation.

**Why the Correct Answer is Right**
When PCO2 increases, it triggers a response to reduce the level of CO2 in the blood. This is achieved through the stimulation of chemoreceptors in the medulla oblongata, which are sensitive to changes in the pH and CO2 levels of the blood. The increased activity of these chemoreceptors sends signals to the respiratory centers in the brain, leading to an increase in the rate and depth of breathing (hyperventilation). This response helps to reduce the level of CO2 in the blood, thereby mitigating the effects of respiratory acidosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because an increase in PCO2 does not lead to a decrease in breathing rate. In fact, the opposite occurs, as the body tries to compensate for the increased CO2 levels by increasing ventilation.
* **Option B:** This option is incorrect because an increase in PCO2 does not lead to a decrease in the production of CO2. The production of CO2 is a metabolic process that occurs in the body's tissues, and it is not directly affected by changes in PCO2 levels.
* **Option C:** This option is incorrect because an increase in PCO2 does not lead to a decrease in the pH of the blood. In fact, the opposite occurs, as the increased CO2 levels lead to a decrease in the pH of the blood, resulting in acidosis.

**Clinical Pearl / High-Yield Fact**
The response to an increase in PCO2 is a classic example of a negative feedback mechanism, where the body tries to compensate for a change in the internal environment (in this case, an increase in CO2 levels) by activating a response that helps to mitigate the effects of that change.

**Correct Answer: C. Increased ventilation.**