**Core Concept:**
Hemoglobin is a protein found in red blood cells, consisting of four chains: two alpha (α) and two beta (β). Its primary function is to transport oxygen (O2) from the lungs to tissues and carbon dioxide (CO2) from tissues back to the lungs. The binding sites for oxygen are located on the globin chains, mainly the heme groups.

**Why the Correct Answer is Right:**
The correct answer is "O2". Hemoglobin has a high affinity for oxygen, meaning it binds oxygen very tightly. In contrast, hemoglobin does not bind or transports all gases except oxygen. This is due to the specific structure and properties of the heme groups that interact with oxygen molecules.

**Why Each Wrong Option is Incorrect:**
A. Carbon dioxide (CO2) binding is not mentioned, but it does bind to hemoglobin, albeit with a lower affinity than O2.
B. Argon (Ar) is a noble gas and does not interfere with the binding of other gases to hemoglobin.
C. Nitrogen (N2) does not bind to hemoglobin, but this is not a gas specifically mentioned in the question.
D. Neon (Ne) is also a noble gas and does not bind to hemoglobin.

**Clinical Pearl / High-Yield Fact:**
Understanding the selective binding properties of hemoglobin is crucial in understanding the pathophysiology of anemia, where a decrease in hemoglobin concentration or altered oxygen-binding affinity can lead to inadequate oxygen delivery to tissues, causing tissue hypoxia and potentially organ damage.

**Correct Answer:**
Hemoglobin does not bind or transports all gases except oxygen (O2).