**Core Concept**
The single pin index safety system is a mechanism designed to prevent incorrect gas cylinder connections, ensuring that the right gas is connected to the right device. This system relies on a specific arrangement of pins and holes on the valve of the gas cylinder and the yoke of the medical equipment.

**Why the Correct Answer is Right**
The single pin index system works by requiring a specific alignment of the pins on the cylinder valve with the corresponding holes on the yoke of the equipment. This precise alignment ensures that the correct gas cylinder is connected, reducing the risk of incorrect gas delivery. The system relies on a specific arrangement of pins and holes, making it impossible to connect a cylinder to the wrong device.

**Why Each Wrong Option is Incorrect**
**Option A:** This option refers to a different type of safety system, the Yoke-type or Twin-pin index system, which has two pins and is not relevant to the question.

**Option B:** This option does not provide a valid alternative to the single pin index system and is therefore incorrect.

**Option C:** This option is not a recognized safety system for gas cylinders and is therefore incorrect.

**Clinical Pearl / High-Yield Fact**
To ensure safe gas cylinder connections, always verify that the single pin index system is engaged before connecting the cylinder to the medical equipment. This can be done by checking that the pins on the cylinder valve align with the corresponding holes on the yoke of the equipment.

**Correct Answer:** D