**Core Concept**
The removal of a general anesthetic gas (G.P.) from the operating room requires careful consideration of the ideal agent for retreatment. This involves understanding the properties of various gases, their effects on patients, and the safety protocols for their use.

**Why the Correct Answer is Right**
The ideal agent for retreatment of G.P. is Nitrous Oxide. This is because Nitrous Oxide has a low blood-gas partition coefficient, which allows for rapid induction and recovery. Additionally, it is non-irritating to the respiratory tract and has a low risk of causing respiratory depression. The low solubility in blood also makes it suitable for rapid removal from the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the properties of the gas that would make it ideal for retreatment.
**Option B:** This option is incorrect because it does not take into account the safety and efficacy of the gas in the context of retreatment.
**Option C:** This option is incorrect because it does not provide any relevant information about the gas and its suitability for retreatment.

**Clinical Pearl / High-Yield Fact**
When considering the removal of a general anesthetic gas, it is essential to choose an agent with low blood-gas partition coefficient, non-irritating properties, and low risk of respiratory depression. This will ensure a safe and rapid recovery for the patient.

**Correct Answer:** D. Nitrous Oxide.