**Core Concept**
The question tests the understanding of anaesthetic gas mixtures, specifically the proportion of oxygen. Anaesthetic machines deliver a mixture of gases, including oxygen, nitrous oxide, and anaesthetic vapors, to ensure patient safety and comfort during surgical procedures. The **oxygen content** in the mixture is crucial for maintaining adequate oxygenation of the patient's blood.

**Why the Correct Answer is Right**
The correct answer, 30%, reflects a common composition of anaesthetic gas mixtures that balances the need for oxygen with the need to minimize the risk of awareness during anaesthesia and to avoid hypoxia. This mixture typically includes **nitrous oxide** and **oxygen**, with the balance being made up by other gases or vapors. The **oxygen concentration** is carefully controlled to meet the patient's metabolic demands while minimizing the risk of anaesthetic complications.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it represents a higher concentration of oxygen than typically used in routine anaesthesia.
**Option B:** is incorrect as it is lower than the usual oxygen content.
**Option D:** is also incorrect as it does not match the standard composition of anaesthetic gas mixtures.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **minimum alveolar concentration (MAC)** of an anaesthetic, which is the concentration needed to prevent movement in 50% of patients in response to a surgical stimulus, is an important concept in anaesthesia. Understanding how different gas mixtures affect MAC and patient oxygenation is critical for safe anaesthetic practice.

**Correct Answer:** Correct Answer: C. 30%.