**Core Concept**
The safety pin index system is a critical safety feature used to prevent incorrect connections of medical gas cylinders, ensuring patient safety and preventing adverse events. This system is based on the **pin index system**, which assigns specific positions to pins on the cylinder valve block. The **oxygen cylinder** has a specific pin configuration to prevent its connection to other gas outlets.

**Why the Correct Answer is Right**
The correct answer corresponds to the specific pin configuration designated for oxygen cylinders. This configuration is standardized to ensure that oxygen cylinders can only be connected to outlets intended for oxygen, thereby minimizing the risk of incorrect gas administration. The pin index system for oxygen cylinders is designed to be unique, preventing the connection of other gas cylinders to oxygen outlets.

**Why Each Wrong Option is Incorrect**
**Option A:** This pin configuration does not match the standardized index for oxygen cylinders.
**Option B:** Similarly, this configuration is not assigned to oxygen cylinders.
**Option C:** This option also does not correspond to the oxygen cylinder's pin index.
**Option D:** This is the correct configuration, so the other options are incorrect by default.

**Clinical Pearl / High-Yield Fact**
Remembering the pin index for common medical gases, including oxygen, is crucial for patient safety. The pin index system is a simple yet effective way to prevent gas cylinder mix-ups, which can have severe consequences.

**Correct Answer:** 2