Which one of the following gases is most soluble in water?
A. CO
B. O₂
C. NO
D. CO₂

To determine which gas is most soluble in water among the given options, we need to consider the nature of the gases and their interactions with water. Here’s a brief overview of each gas:

1. **Carbon Monoxide (CO)**: This is a nonpolar molecule and has limited solubility in water. It does not react with water to form any additional compounds.

2. **Oxygen (O₂)**: This is also a nonpolar molecule. Its solubility in water is relatively low because it does not interact strongly with water molecules.

3. **Nitric Oxide (NO)**: This gas is somewhat soluble in water. It can react with water to form nitrous acid (HNO₂), which may increase its overall solubility compared to other nonpolar gases.

4. **Carbon Dioxide (CO₂)**: This gas is more soluble in water than the previous gases. It reacts with water to form carbonic acid (H₂CO₃), which increases its solubility. The reaction can be represented as:
\[
\text{CO}_2(g) + \text{H}_2\text{O}(l) \leftrightarrow \text{H}_2\text{CO}_3(aq)
\]

### Conclusion

Among the four gases, **Carbon Dioxide (CO₂)** is the most soluble in water due to its ability to form carbonic acid upon dissolution, which significantly enhances its solubility compared to the other gases listed.

Therefore, the answer is **D. CO₂**.