### Core Concept
The semi-circular canals are a part of the vestibular apparatus in the inner ear, responsible for detecting rotational movements and maintaining balance. They contain a fluid called endolymph and are lined with sensory hair cells that respond to changes in angular acceleration.

### Why the Correct Answer is Right
Angular acceleration is detected by the semi-circular canals through the movement of the fluid within them. As the head rotates, the fluid lags behind due to inertia, causing the hair cells to bend and send signals to the brain. This information is crucial for maintaining balance and preventing the sensation of spinning. The semi-circular canals are specifically designed to detect changes in angular acceleration, which is distinct from linear acceleration, static equilibrium, or auditory processing.

### Why Each Wrong Option is Incorrect
**Option A:** Linear acceleration is detected by the otolith organs (utricle and saccule), not the semi-circular canals. Linear acceleration refers to movement in a straight line.

**Option C:** Static equilibrium is maintained by the otolith organs (utricle and saccule) and the vestibular nuclei, but not by the semi-circular canals. Static equilibrium refers to the ability to maintain balance when stationary.

**Option D:** Hearing is primarily detected by the cochlea, not the semi-circular canals. The cochlea contains the auditory nerve and is responsible for sound processing.

### Clinical Pearl / High-Yield Fact
A key clinical correlation is that damage to the semi-circular canals can lead to benign paroxysmal positional vertigo (BPPV), a condition characterized by intense, brief episodes of vertigo triggered by specific head movements.

### Correct Answer Line
✓ Correct Answer: B. Angular acceleration.