**Core Concept:** Pacemaker regulating the rate of respiration involves the respiratory centre in the medulla oblongata. The respiratory centre controls both the rate and depth of breathing.

**Why the Correct Answer is Right:** The correct answer, **C:** the respiratory centre in the medulla oblongata, is right because it is responsible for regulating the rate of respiration. The respiratory centre is a group of neurons located in the medulla oblongata, which is part of the brainstem. It receives sensory input from various sources, including arterial blood gases, carbon dioxide levels, and chemical and mechanical stimuli, and integrates this information to modulate respiratory rate based on the body's needs.

**Why Each Wrong Option is Incorrect:**

A. **Brainstem**: While the respiratory centre is indeed located in the brainstem, the correct answer should be the respiratory centre specifically in the medulla oblongata.

B. **Diaphragm**: The diaphragm is crucial for respiratory movement, but it is not responsible for regulating the rate of respiration. The respiratory centre in the medulla oblongata does that.

D. **Cerebral Cortex**: The cerebral cortex is involved in higher cognitive functions and does not regulate the rate of respiration. This control occurs in the respiratory centre located in the medulla oblongata.

**Clinical Pearl / High-Yield Fact:** Understanding the respiratory centre's role in regulating the rate of respiration is essential in understanding respiratory disorders like respiratory depression or hyperventilation. These conditions can result from medications, anesthesia, or pathological processes affecting the respiratory centre in the medulla oblongata.

**Correct Answer:** C. Respiratory centre in the medulla oblongata.