## Core Concept
Pneumoperitoneum is a technique used in laparoscopic surgery where a gas is insufflated into the abdominal cavity to create a clear visual and operative space. The choice of gas is critical for safety and efficacy. The ideal gas should be colorless, non-flammable, and have low blood solubility to minimize complications.

## Why the Correct Answer is Right
The correct answer, **CO2**, is widely used for pneumoperitoneum because it does not support combustion, is rapidly absorbed and cleared from the body, and has a low risk of causing gas embolism compared to other gases. Its high blood solubility allows for quick absorption and reduction of intra-abdominal pressure if needed. CO2 is also inexpensive and readily available.

## Why Each Wrong Option is Incorrect
- **Option A: O2** - Oxygen supports combustion and could increase the risk of fire during surgical procedures, especially when electrocautery is used.
- **Option B: Air** - Air contains 79% nitrogen, which is poorly soluble in blood. This could lead to complications like gas embolism. Also, air contains oxygen, posing a combustion risk.
- **Option D: Helium** - Although helium is inert and does not support combustion, it has low blood solubility, which increases the risk of gas embolism. It's also more expensive than CO2.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **CO2** is the gas of choice for laparoscopic procedures due to its favorable properties. A classic exam trap could be questions about the risks associated with using alternative gases, so understanding the properties of CO2 and why it's preferred is crucial.

## Correct Answer Line
**Correct Answer: C. CO2**.