**Core Concept**
The Nernst potential for a particular ion is the equilibrium potential at which the concentration gradient of that ion is balanced by the electrical gradient, resulting in no net movement of the ion across the membrane. In the case of potassium (K+), the Nernst potential is determined by the concentration of K+ ions outside and inside the cell.

**Why the Correct Answer is Right**
The Nernst potential for K+ is calculated using the Nernst equation, which takes into account the concentration of K+ ions outside and inside the cell. Typically, the concentration of K+ ions is higher outside the cell than inside, resulting in a negative Nernst potential for K+. The Nernst potential for K+ is approximately -90 mV, which means that at this potential, there will be no net movement of K+ ions across the membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Nernst potential for K+ is not determined by the concentration of Na+ ions.
**Option B:** This option is incorrect because the Nernst potential for K+ is not equal to the resting membrane potential.
**Option C:** This option is incorrect because the Nernst potential for K+ is not dependent on the concentration of Cl- ions.

**Clinical Pearl / High-Yield Fact**
The Nernst potential for K+ is an important concept in understanding the resting membrane potential and the behavior of neurons and other excitable cells. A fundamental understanding of the Nernst potential for K+ is essential for appreciating the mechanisms underlying various physiological and pathological processes.

**Correct Answer: B. -90 mV**