**Core Concept**
The Nernst potential is a measure of the electrical potential difference across a cell membrane, resulting from the diffusion of ions down their concentration gradient. In the context of potassium (K+), the Nernst potential is calculated based on the concentration gradient of K+ ions across the cell membrane and the permeability of the membrane to K+.

**Why the Correct Answer is Right**
The Nernst potential for K+ is calculated using the Nernst equation: E = (RT/F) * ln([K+]_o / [K+]_i), where E is the Nernst potential, R is the gas constant, T is the temperature in Kelvin, F is the Faraday constant, [K+]_o is the concentration of K+ outside the cell, and [K+]_i is the concentration of K+ inside the cell. Typically, the concentration of K+ is higher outside the cell than inside, which results in a negative Nernst potential for K+. The value of -90mV is commonly cited for the resting membrane potential of neurons, which is close to the calculated Nernst potential for K+.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The Nernst potential for K+ is a critical component of the resting membrane potential, which is essential for the proper functioning of neurons and other excitable cells. A disruption in the balance of K+ ions across the cell membrane can lead to abnormal electrical activity, such as in conditions like hyperkalemia or hypokalemia.

**Correct Answer: -90mV**