**Core Concept**
The membrane potential of hair cells in the inner ear is a critical aspect of sound processing and balance. Hair cells are specialized sensory cells that convert mechanical vibrations into electrical signals. The resting membrane potential of hair cells is generated by a combination of ionic gradients and ion channels.

**Why the Correct Answer is Right**
The correct answer is related to the ionic composition of the endolymph, a fluid that bathes the hair cells in the inner ear. The endolymph has a high concentration of potassium ions (K+), which contributes to the negative resting membrane potential of hair cells. The hair cells have a high permeability to potassium ions, leading to an efflux of K+ ions out of the cell, resulting in a negative membrane potential. This negative potential is essential for the depolarization of hair cells in response to sound vibrations.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the resting membrane potential of hair cells is not primarily generated by sodium-potassium pumps.
* **Option B:** This option is incorrect because the membrane potential of hair cells is not determined by the concentration of chloride ions in the endolymph.
* **Option C:** This option is incorrect because the membrane potential of hair cells is not equal to the Nernst potential for potassium ions.

**Clinical Pearl / High-Yield Fact**
The hair cells in the inner ear are responsible for converting mechanical vibrations into electrical signals, which are then transmitted to the auditory nerve and interpreted by the brain as sound.

**Correct Answer:** A. Negatively charged, approximately -70 mV.