**Core Concept:** Gas therapy in medicine typically involves converting gases into a liquid form to facilitate storage, transportation, and administration. The process of liquefying gases is known as **vaporisation**.

**Why the Correct Answer is Right:** In the context of the question, the correct answer refers to the process of liquefying gases, which is crucial for their safe storage and administration. Among the given options, the correct answer pertains to the liquefaction of **Carbon dioxide (CO2)**, which is essential for the safe handling and delivery of CO2 in medical procedures like anesthesia, respiratory therapy, and neonatal resuscitation.

**Why Each Wrong Option is Incorrect:**
A. Oxygen (O2): Oxygen is typically administered in its gaseous form due to its high solubility in blood and tissues, making liquefaction unnecessary.
B. Nitrous oxide (N2O): Nitrous oxide is also administered in its gaseous form, as it has low solubility in blood and tissues. Liquefying it would not provide any advantage in practice.
C. Nitrous oxide (N2O): As mentioned earlier, nitrous oxide is administered in its gaseous form and does not require liquefaction.
D. Halothane: Halothane is a general anesthetic agent, and like nitrous oxide, it is administered in its gaseous form due to its low solubility in blood and tissues.

**Clinical Pearl:** In anesthesia practice, liquefying gases like CO2 is vital for safe handling and administration in procedures involving respiratory support or neonatal resuscitation. The correct answer, CO2, highlights the importance of gas liquefaction in maintaining patient safety during these procedures.

**Correct Answer:** CO2.