**Core Concept**
The level of resting ventilation is primarily controlled by the respiratory centers in the brainstem, which receive inputs from various stimuli. The most important stimulus controlling resting ventilation is the level of carbon dioxide (CO2) in the blood, specifically the partial pressure of CO2 (PaCO2).

**Why the Correct Answer is Right**
The level of CO2 in the blood is detected by chemoreceptors in the carotid bodies and medulla oblongata. These chemoreceptors send signals to the respiratory centers, which adjust the rate and depth of breathing to maintain a stable level of CO2. The respiratory centers use a negative feedback mechanism to regulate ventilation, increasing ventilation when PaCO2 rises and decreasing it when PaCO2 falls. This mechanism is essential for maintaining acid-base balance and ensuring proper oxygenation of the body's tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, while pH levels in the blood can affect ventilation, they are not the primary stimulus controlling resting ventilation.
**Option B:** This option is incorrect because, although oxygen levels in the blood can affect ventilation, they are not the primary stimulus controlling resting ventilation.
**Option C:** This option is incorrect because, while temperature can affect ventilation, it is not the primary stimulus controlling resting ventilation.

**Clinical Pearl / High-Yield Fact**
The level of CO2 in the blood is a critical determinant of resting ventilation, and changes in PaCO2 can have significant effects on respiratory function. It is essential to understand the relationship between CO2 levels and ventilation to diagnose and manage respiratory disorders.

**Correct Answer: C. The level of carbon dioxide in the blood.**