**Core Concept**
The respiratory system is controlled by a complex interplay of neurons in the medulla oblongata, which regulate breathing rate and depth. This control is largely governed by the sensitivity of respiratory neurons to changes in blood pH, carbon dioxide (CO2) levels, and oxygen (O2) levels. The chemoreceptors and baroreceptors in the carotid and aortic bodies, as well as the central chemoreceptors in the medulla, play crucial roles in detecting these changes and modulating respiratory neuron activity.

**Why the Correct Answer is Right**
A rise in Po2 (partial pressure of oxygen) would generally be expected to decrease respiratory neuron activity, not increase it. This is because high oxygen levels are associated with adequate oxygenation of the blood and tissues. In contrast, a rise in PCO2 (partial pressure of carbon dioxide) and H+ concentration (increase in acidity) would stimulate respiratory neuron activity, as they indicate inadequate ventilation and the need for increased breathing to remove CO2 and alleviate acidosis. A drop in Po2 would also stimulate respiratory neuron activity, as it indicates inadequate oxygenation and the need for increased breathing to take in more oxygen.

**Why Each Wrong Option is Incorrect**
**Option A:** A rise in PCO2 would indeed increase respiratory neuron activity, as it indicates that the body needs to increase ventilation to remove excess CO2 and alleviate acidosis.

**Option B:** A rise in H+ concentration (increase in acidity) would also increase respiratory neuron activity, as it indicates that the body needs to increase ventilation to remove excess CO2 and alleviate acidosis.

**Option D:** A drop in Po2 would increase respiratory neuron activity, as it indicates inadequate oxygenation and the need for increased breathing to take in more oxygen.

**Clinical Pearl / High-Yield Fact**
The central chemoreceptors in the medulla are particularly sensitive to changes in blood pH, and can detect even small changes in H+ concentration. This is in contrast to the peripheral chemoreceptors, which are more sensitive to changes in Po2 and PCO2.

**✓ Correct Answer:** C. Rise in Po2