**Core Concept**
The question is testing the knowledge of anaesthetic agents and their effects on pain perception. The correct answer is related to the mechanism of action of a specific anaesthetic agent that causes analgesia.

**Why the Correct Answer is Right**
Nitrous oxide is a widely used anaesthetic agent that causes analgesia through its action on the central nervous system. It acts as a N-methyl-D-aspartate (NMDA) receptor antagonist, which inhibits the release of excitatory neurotransmitters such as glutamate. This leads to a decrease in the transmission of pain signals to the brain, resulting in analgesia. Nitrous oxide also has a depressant effect on the respiratory and cardiovascular systems, which is responsible for its anaesthetic properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify an anaesthetic agent that causes analgesia. While some inhalational anaesthetics have analgesic properties, they are not as potent as nitrous oxide.
**Option B:** This option is incorrect because it refers to a muscle relaxant, not an anaesthetic agent. Muscle relaxants are used to facilitate endotracheal intubation and to relax skeletal muscles, but they do not cause analgesia.
**Option C:** This option is incorrect because it refers to a local anaesthetic, not a general anaesthetic agent. Local anaesthetics are used to numb a specific area of the body and do not cause general analgesia.

**Clinical Pearl / High-Yield Fact**
It's worth noting that nitrous oxide is a commonly used anaesthetic agent in dental and minor surgical procedures due to its rapid onset and short duration of action. However, it can cause nausea and vomiting as side effects.

**Correct Answer:** D. Nitrous oxide