Step 1: What the question is about.
We want to know which structure first notices a rise in carbon dioxide in the blood and reacts to it.
Step 2: How the body senses blood gases.
The body has special sensors called chemoreceptors. They sit in the carotid bodies and the aortic arch. Their job is to keep checking the levels of $CO_2$ and $H^+$ (acidity) in the blood.
Step 3: What happens when $CO_2$ goes up.
When $CO_2$ rises, the blood becomes a little more acidic. The chemoreceptors pick this up at once and send signals to the respiratory centres so that breathing speeds up and the extra $CO_2$ is thrown out.
Step 4: Why the other choices are wrong.
The inspiratory and expiratory centres are control areas in the brain that get the message and act on it. They do not directly sense the gas. Baroreceptors only sense blood pressure, not $CO_2$.
Step 5: Putting it together.
So the very first response to high $CO_2$ comes from the chemoreceptors.
Step 6: Conclusion.
Therefore the correct answer is "Chemoreceptors". \[ \boxed{\text{Chemoreceptors}} \]