**Core Concept**
The question is testing the student's knowledge of the effects of different general anaesthetics on the respiratory system, specifically their impact on airway smooth muscle tone. **Bronchodilation** refers to the widening of the airways, which can be beneficial for patients with respiratory conditions such as asthma.

**Why the Correct Answer is Right**
Halothane is a volatile anaesthetic that causes relaxation of smooth muscle in the airways, leading to **bronchodilation**. This effect is due to the inhibition of calcium channels in smooth muscle cells, reducing the release of intracellular calcium and subsequent contraction of the muscle. Halothane's bronchodilatory effect makes it a preferred choice for patients with asthma or chronic obstructive pulmonary disease (COPD).

**Why Each Wrong Option is Incorrect**
* **Option A:** Isoflurane is another volatile anaesthetic, but it has a minimal effect on airway smooth muscle tone and is not known for causing significant bronchodilation.
* **Option B:** Propofol is a non-volatile anaesthetic that can cause respiratory depression, but it does not have a significant bronchodilatory effect.
* **Option D:** Ketamine can cause airway irritation and bronchospasm in some patients, so it is not the best choice for bronchodilation.

**Clinical Pearl / High-Yield Fact**
When choosing an anaesthetic for patients with respiratory disease, consider the pharmacological properties of each agent and select one that is likely to cause bronchodilation, such as halothane or isoflurane.

**Correct Answer:** A. Halothane.