**Core Concept**
The resting membrane potential of cardiac muscle is a critical concept in understanding the electrophysiology of the heart. It is the difference in electrical potential between the inside and outside of a cardiac muscle cell at rest, measured in millivolts. The resting membrane potential is primarily generated by the movement of ions, particularly sodium (Na+) and potassium (K+), across the cardiac cell membrane.

**Why the Correct Answer is Right**
The resting membrane potential of cardiac muscle is approximately -85 millivolts. This value is primarily determined by the balance between the inward movement of positively charged sodium ions and the outward movement of positively charged potassium ions. The cardiac cell membrane is more permeable to potassium ions than to sodium ions at rest, resulting in a net outward flow of positively charged ions, which creates a negative charge inside the cell. The resting membrane potential is also influenced by the presence of ion pumps, such as the sodium-potassium pump, which helps to maintain the ion balance across the cell membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot assess its correctness.
**Option B:** Incorrect, as the resting membrane potential of cardiac muscle is not approximately 0 millivolts.
**Option C:** Incorrect, as the resting membrane potential of cardiac muscle is not approximately +85 millivolts.

**Clinical Pearl / High-Yield Fact**
The resting membrane potential of cardiac muscle is a critical determinant of the heart's excitability and contractility. Abnormalities in the resting membrane potential can lead to arrhythmias and other cardiac conduction disorders.

**Correct Answer: D. -85 millivolts**