**Core Concept**
The pin index safety system is a mechanism designed to prevent the incorrect connection of medical gas cylinders. Each gas cylinder has a unique pin configuration that corresponds to a specific outlet on an anesthesia machine or other medical device. The pin index system helps prevent accidental gas mix-ups and ensures that patients receive the correct gas for their medical treatment.

**Why the Correct Answer is Right**
Nitrous oxide (N2O) has a specific pin configuration that matches a particular outlet on an anesthesia machine. The correct pin index for N2O is 3, indicating that the cylinder has three pins in a specific arrangement. This configuration ensures that the N2O cylinder can only be connected to the correct outlet on the anesthesia machine, preventing accidental gas mix-ups and ensuring patient safety.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the correct pin configuration for N2O. The pin index for oxygen (O2) is typically 1, not the option listed here.
**Option B:** This option is also incorrect because it does not match the correct pin configuration for N2O. The pin index for air is typically 0, not the option listed here.
**Option C:** This option is incorrect because it does not match the correct pin configuration for N2O. The pin index for other medical gases, such as nitrous oxide, may differ from the one listed here.

**Clinical Pearl / High-Yield Fact**
The pin index safety system is an important mechanism for preventing medical gas mix-ups and ensuring patient safety. It is essential for medical professionals to understand the correct pin configurations for different medical gases to prevent errors and ensure accurate patient treatment.

**Correct Answer:** C. 3