**Core Concept**
The regulation of breathing is a complex process involving various receptors and pathways. The increase in CO2 levels is a potent stimulator of ventilation. **Chemoreceptors** play a crucial role in detecting changes in CO2 levels.

**Why the Correct Answer is Right**
CO2 increases ventilation by acting mainly on **chemoreceptors** located in the carotid and aortic bodies. These receptors are sensitive to changes in the partial pressure of CO2, as well as pH levels. When CO2 levels increase, it stimulates the **chemoreceptors** to send signals to the brain, which in turn increases ventilation to eliminate the excess CO2.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while **baroreceptors** are involved in regulating blood pressure, they are not the primary receptors responsible for detecting changes in CO2 levels.
**Option B:** This option is incorrect because **mechanoreceptors** are involved in detecting changes in lung volume and pressure, but not directly in responding to CO2 levels.
**Option D:** This option is not provided, but typically, other options might include receptors not primarily involved in CO2 detection.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **chemoreceptors** are key players in the regulation of breathing in response to CO2 levels. An increase in CO2 is a potent stimulator of ventilation, and this mechanism is vital for maintaining acid-base balance.

**Correct Answer:** D. Chemoreceptors.