**Core Concept**
The primary receptor cells of hearing are responsible for converting sound vibrations into electrical signals that can be interpreted by the brain. This process occurs in the **cochlea**, a spiral-shaped structure in the inner ear. The **mechanoreceptors** in the cochlea play a crucial role in this conversion.

**Why the Correct Answer is Right**
The correct answer is related to the function of **hair cells** in the cochlea. These **mechanoreceptor cells** are embedded in the **basilar membrane** and are responsible for detecting sound-induced vibrations. When sound waves reach the cochlea, they cause the **basilar membrane** to vibrate, which in turn bends the **stereocilia** on the **hair cells**, triggering an electrical signal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary receptor cells of hearing.
**Option B:** This option is also incorrect as it does not relate to the cochlea or the process of hearing.
**Option C:** Similarly, this option is incorrect as it is not associated with the conversion of sound vibrations into electrical signals.
**Option D:** This option is incorrect because it is not related to the **mechanoreceptors** in the cochlea.

**Clinical Pearl / High-Yield Fact**
The **hair cells** in the cochlea are highly sensitive to sound-induced vibrations, and damage to these cells can result in **sensorineural hearing loss**.

**Correct Answer:** D. Hair cells.