## Core Concept
The pin index system is a safety feature designed to prevent incorrect cylinder attachments by having specific positions for pins on the yoke of anesthesia machines that correspond to holes on the cylinder heads. This system is crucial for ensuring patient safety by preventing the administration of incorrect gases.

## Why the Correct Answer is Right
The correct pin index for oxygen cylinders is **2, 5**. This specific configuration ensures that only oxygen cylinders can be attached to the corresponding yoke on anesthesia machines, thereby preventing mix-ups with other gases. The pin index system is standardized and critical for safe anesthesia practice.

## Why Each Wrong Option is Incorrect
- **Option A:** 1, 5 - This is incorrect because the pin index for oxygen is specifically defined as 2, 5, not 1, 5.
- **Option B:** 3, 5 - This is incorrect as it does not match the standardized pin index for oxygen cylinders.
- **Option D:** 1, 6 - This is incorrect because it corresponds to a different gas cylinder, not oxygen.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the pin index system varies by country, but for oxygen in many countries, including those following international standards, the index is **2, 5**. It's essential to familiarize oneself with the specific pin indexes for commonly used medical gases to ensure safe practice.

## Correct Answer: C. 2, 5.