**Core Concept:**
The horizontal semicircular canal is one of the three canals in the human vestibular system, responsible for detecting changes in linear acceleration, particularly in the horizontal plane. When stimulated, it triggers a reflex called the vestibulo-ocular reflex (VOR), which aims to stabilize the image on the retina by moving the eyes in the opposite direction to the head movement.

**Why the Correct Answer is Right:**
When the horizontal semicircular canal is stimulated, it triggers the VOR to counteract the head movement by moving the eyes in the opposite direction. In this case, the stimulated canal detects an increase in linear acceleration in the horizontal plane, and to maintain gaze stability, the eyes will move in the opposite direction to counteract the head movement. This results in a nystagmus (rapid, involuntary eye movements) with the slow phase moving in the direction of the head movement and the quick phase moving in the opposite direction.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because when the horizontal semicircular canal is stimulated, the eyes should move in the opposite direction to counteract the head movement, resulting in a nystagmus with the slow phase moving in the direction of head movement and the quick phase moving in the opposite direction.
B. This option is incorrect because nystagmus is a reflexive response triggered by the vestibular system to counteract head movements. Stimulation of the horizontal semicircular canal results in nystagmus with the slow phase moving in the direction of head movement and the quick phase moving in the opposite direction.
C. This option is incorrect because nystagmus is a reflexive response triggered by the vestibular system to counteract head movements. Stimulation of the horizontal semicircular canal results in nystagmus with the slow phase moving in the direction of head movement and the quick phase moving in the opposite direction.
D. This option is incorrect because nystagmus is a reflexive response triggered by the vestibular system to counteract head movements. Stimulation of the horizontal semicircular canal results in nystagmus with the slow phase moving in the direction of head movement and the quick phase moving in the opposite direction.

**Clinical Pearl:** Nystagmus is a vital tool for maintaining gaze stability and balance during head movements. The direction of nystagmus reflects the direction of head movement, with the slow phase moving in the direction of the head movement and the quick phase moving in the opposite direction. This helps the brain to accurately perceive the direction of the head motion and adjust the eye movements accordingly to keep the visual field on the target.