**Core Concept**
The cochlea, a spiral-shaped structure in the inner ear, is responsible for sound processing and conversion into electrical signals that are transmitted to the brain. The process of sound-induced depolarization in the cochlea involves the movement of the basilar membrane and the opening of mechanoreceptors called hair cells.

**Why the Correct Answer is Right**
The correct answer involves the stimulation of the inner hair cells by the movement of the basilar membrane, which leads to the opening of mechanically gated ion channels. This results in an influx of potassium ions (K+) into the hair cell, causing depolarization. The depolarized hair cells then release neurotransmitters that stimulate the afferent auditory nerve fibers. This process is crucial for the transmission of sound signals to the brain.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the depolarization of the cochlea is not primarily due to the movement of the stapes bone, which is a part of the ossicular chain involved in sound transmission.
* **Option B:** This option is incorrect because the depolarization of the cochlea is not directly related to the activity of the olivocochlear bundle, which is a group of efferent nerve fibers that modulate the activity of the cochlea.
* **Option C:** This option is incorrect because the depolarization of the cochlea is not primarily due to the movement of the round window membrane, which is a flexible membrane that helps to regulate the pressure in the cochlea.

**Clinical Pearl / High-Yield Fact**
The depolarization of the cochlea is a complex process that involves the coordinated movement of the basilar membrane and the opening of mechanically gated ion channels in the hair cells. Understanding this process is crucial for diagnosing and managing hearing disorders, such as sensorineural hearing loss.

**Correct Answer: C. Movement of the basilar membrane.**