## Core Concept
The Pin Index Safety System (PISS) is a safety feature designed to prevent incorrect cylinder attachments by having specific pin configurations for different medical gases. This system helps prevent accidents and ensures the correct gas is used for medical procedures.

## Why the Correct Answer is Right
The correct pin index for nitrous oxide cylinders is **3 and 5**. This specific configuration is part of the Pin Index Safety System, which assigns unique pin positions to different gas cylinders to prevent mix-ups. The index is crucial for ensuring patient safety during medical procedures.

## Why Each Wrong Option is Incorrect
* **Option A:** 1 and 5 - This is incorrect because it does not match the designated pin index for nitrous oxide.
* **Option B:** 2 and 5 - This is incorrect as it is actually the pin index for oxygen cylinders.
* **Option D:** 6 - This option is incomplete and incorrect as it does not specify the second pin index.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the pin index system varies by country, but for nitrous oxide in many countries, including those following international standards, the pin index is **3 and 5**. Always verify the specific regulations and standards in your region.

## Correct Answer: C. 3 and 5.