**Core Concept**
The resting membrane potential (RMP) of a neuron is primarily determined by the movement of potassium (K+) ions across the cell membrane. The RMP is established by the balance between potassium efflux and the electrochemical gradient generated by the sodium-potassium pump.

**Why the Correct Answer is Right**
When the extracellular concentration of K+ is decreased, the electrochemical gradient for potassium efflux becomes more favorable. This means that potassium ions are more likely to flow out of the cell, leading to a more negative RMP. The magnitude of the RMP increases due to the increased potassium efflux, resulting in a more negative membrane potential.

**Why Each Wrong Option is Incorrect**
* **Option A:** A decrease in extracellular K+ would not lead to a decrease in the magnitude of RMP; instead, it would lead to an increase.
* **Option B:** A decrease in extracellular K+ would make the membrane more negative, not less negative.
* **Option C:** A decrease in extracellular K+ would lead to an increase in the magnitude of RMP, not a decrease.

**Clinical Pearl / High-Yield Fact**
The resting membrane potential is a critical determinant of neuronal excitability. Changes in the extracellular concentration of K+ can significantly impact the RMP, leading to changes in neuronal excitability and potentially affecting various physiological and pathological processes.

**Correct Answer: D. ↓negativity of membrane**