**Core Concept:**
Resting membrane potential (RMP) is the electrochemical gradient maintained by neurons, muscle cells, and other excitable cells, which prevents the cell from depolarizing uncontrollably. It plays a crucial role in the generation and propagation of action potentials and cellular excitability. In the context of skeletal muscle cells, the resting membrane potential is maintained by the balance of ion channels, pumps, and transporters across the cell membrane.

**Why the Correct Answer is Right:**
The resting membrane potential of skeletal muscle is equal to the equilibrium potential of potassium ions (K+). In these cells, the K+ gradient is maintained by the Na+/K+-ATPase pump, which actively transports three Na+ ions out of the cell for every two K+ ions taken in. This maintains a higher concentration of K+ ions outside the cell, creating a K+ gradient. As a result, the equilibrium potential of K+ is also higher outside the cell.

**Why Each Wrong Option is Incorrect:**
A. Equilibrium potential of chloride ions (Cl-) is incorrect because chloride ion concentration gradient is maintained by the Cl-/HCO3- and Cl-/OH- transporters, which are not directly related to the resting membrane potential.
B. Equilibrium potential of sodium ions (Na+) is incorrect because sodium ions are actively transported out of the cell by the Na+/K+-ATPase pump, which does not contribute to resting membrane potential.
C. Equilibrium potential of calcium ions (Ca2+) is incorrect because calcium ions (Ca2+) are actively transported out of the cell by the Ca2+-ATPase pump, which also does not contribute to resting membrane potential.

**Clinical Pearl:**
Understanding the resting membrane potential and its relation to ion concentrations is essential for understanding neuronal excitability, muscle contraction, and cellular responses to external stimuli.

**Correct Answer:** Equilibrium potential of potassium ions (K+)

**Explanation:** The resting membrane potential of skeletal muscle cells is determined by the concentration gradient of potassium ions (K+). The Na+/K+-ATPase pump actively transports three Na+ ions out of the cell for every two K+ ions taken in, creating a higher concentration of K+ ions outside the cell and, thus, a higher equilibrium potential for K+ ions. This K+ gradient and resulting equilibrium potential contribute to the maintenance of skeletal muscle cell membrane stability and excitability.