**Core Concept:** Subarachnoid blockade is a type of regional anaesthesia where a local anaesthetic is injected into the subarachnoid space, which surrounds the spinal cord and brainstem. This results in a loss of sensation and motor function below the level of block. The question is asking about the plane of anaesthesia where motor function in the head and neck is lost due to blockade of the oculomotor nerves.

**Why the Correct Answer is Right:** The correct answer, **D. Cervical (C1-C2)**, refers to the plane of anaesthesia where the oculomotor nerves are blocked, leading to loss of movement of the eyeballs. The oculomotor nerves (CN III) arise from the brainstem and supply muscles responsible for eye movement. The cervical region (C1-C2) is where the spinal cord meets the brainstem, making it the appropriate plane for blockade to affect the oculomotor nerves.

**Why Each Wrong Option is Incorrect:**

A. **Thoracic (T1-T6)**: This option targets the sympathetic chain and paravertebral nerves, not the oculomotor nerves.
B. **Lumbar (L1-L4)**: This option targets the sacral plexus, which is not related to eye movement.
C. **Low Thoracic (T7-T10)**: This option targets sympathetic chain and intercostal nerves, not directly related to oculomotor nerves.

**Clinical Pearl:** Understanding the anatomy and planes of regional anaesthesia is crucial for safe and effective surgical anaesthesia. This concept can be applied not only to oculomotor nerves but also to other nerves and their respective blocks.

**Correct Answer:** .