**Core Concept**
The partial pressure of carbon dioxide in exhaled breath (ETCO2) is an indirect measure of the partial pressure of carbon dioxide in arterial blood (PaCO2). It is influenced by the rate and depth of breathing, as well as the respiratory and metabolic states of the individual.

**Why the Correct Answer is Right**
During increased rate and depth of breathing, the lungs are able to remove more CO2 from the blood, resulting in a decrease in ETCO2. This occurs because the increased ventilation allows for a greater exchange of gases between the alveoli and the bloodstream. The rate and depth of breathing influence the rate of CO2 removal from the blood, which in turn affects ETCO2 levels. This is a fundamental principle of respiratory physiology.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because increasing the rate and depth of breathing would actually decrease ETCO2, not increase it.

**Option B:** This option is incorrect because ETCO2 is not directly related to the respiratory rate or depth in a non-linear fashion.

**Option C:** This option is incorrect because ETCO2 is not directly related to the respiratory rate or depth in a non-linear fashion.

**Clinical Pearl / High-Yield Fact**
In clinical practice, ETCO2 is often used as a non-invasive indicator of ventilatory status, particularly in patients who are critically ill or undergoing mechanical ventilation. A decrease in ETCO2 can indicate increased ventilation or decreased CO2 production, while an increase in ETCO2 can indicate decreased ventilation or increased CO2 production.

**Correct Answer: D. Decrease**