## Core Concept
The question tests the knowledge of gases that are stored in liquid form, which is a critical concept in pharmacology and medical practice. Certain gases are stored in liquid form under pressure to facilitate their use in medical settings. This method of storage is essential for gases that are used frequently or in large quantities.

## Why the Correct Answer is Right
The correct answer, **Nitrous oxide**, is stored in liquid form because it can be liquefied under moderate pressure at room temperature. Nitrous oxide cylinders contain a mixture of liquid and vapor phases. As the vapor is used, the liquid phase evaporates to replace it, maintaining a constant pressure within the cylinder until the liquid phase is depleted. This property makes nitrous oxide suitable for storage in liquid form for medical and dental procedures.

## Why Each Wrong Option is Incorrect
- **Option A:** While specific details about the gas are not provided, we can infer that not all gases are suitable for storage in liquid form under standard conditions. Without the specific gas listed, we focus on the properties of commonly used medical gases.
- **Option B:** Similarly, without specifics, we consider general properties; many gases are not stored in liquid form due to their critical temperatures and pressures.
- **Option D:** This option is incorrect because, typically, gases like oxygen are stored under high pressure in gaseous form, not liquid form, due to their physical properties.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **nitrous oxide** is used not only for its anesthetic properties but also for its analgesic effects. It's commonly used in dental procedures for sedation and pain relief. Understanding the storage and usage of medical gases is crucial for safe and effective patient care.

## Correct Answer: C. Nitrous oxide.