**Core Concept**
The question requires knowledge of chemoreceptors, which are specialized sensory receptors that detect changes in the chemical composition of the blood. Chemoreceptors play a crucial role in regulating breathing and cardiovascular function by responding to alterations in the levels of oxygen, carbon dioxide, and hydrogen ions in the blood.

**Why the Correct Answer is Right**
The carotid body is a chemoreceptor located near the bifurcation of the common carotid artery. It contains type I (glomus) cells, which respond to decreases in oxygen levels and increases in carbon dioxide and hydrogen ion concentrations. When stimulated, these cells release neurotransmitters that signal the brain to increase breathing rate and depth. The aortic body, another chemoreceptor, is located along the aorta and responds to similar changes in blood chemistry.

**Why Each Wrong Option is Incorrect**
* **Option A:** The sinoatrial (SA) node is the natural pacemaker of the heart, responsible for generating electrical impulses that regulate heartbeat. It is not a chemoreceptor.
* **Option B:** The baroreceptors are stretch receptors located in the walls of blood vessels, particularly in the carotid sinus and aortic arch. They respond to changes in blood pressure and play a key role in regulating cardiac output and vascular tone, but are not chemoreceptors.
* **Option D:** The pulmonary stretch receptors are located in the bronchi and bronchioles of the lungs and respond to lung inflation. They help regulate breathing by sending signals to the brain to decrease breathing rate when the lungs are overinflated.

**Clinical Pearl / High-Yield Fact**
Chemoreceptors are sensitive to changes in blood pH, and their stimulation can lead to respiratory alkalosis, as the body attempts to compensate for the perceived increase in blood CO2 levels by increasing ventilation.

**Correct Answer: C. Carotid body. Carotid body.**