**Core Concept**
The **Nernst equation** for potassium (**K+**) calculates the equilibrium potential for **K+** ions based on their concentration gradient across the cell membrane. The **resting membrane potential** is largely influenced by the movement of **K+** ions.

**Why the Correct Answer is Right**
A decrease in extracellular **K+** concentration increases the negativity of the **equilibrium potential for K+**, making the **membrane potential** more negative. This is because the **Nernst equation** shows that a decrease in extracellular **K+** concentration decreases the ratio of extracellular to intracellular **K+** concentration, resulting in a more negative **equilibrium potential**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a decrease in extracellular **K+** does not directly cause membrane depolarization.
**Option B:** Incorrect as it's not the direct effect of decreased extracellular **K+** on the membrane.
**Option C:** Incorrect because the effect of decreased extracellular **K+** is not to make the membrane potential less negative.
**Option D:** Incorrect as decreased extracellular **K+** does not directly cause an increase in membrane excitability in this context.

**Clinical Pearl / High-Yield Fact**
Remember, the **Nernst equation** helps predict how changes in ion concentrations affect membrane potential, which is crucial for understanding various physiological and pathological states, including **hypokalemia** and its effects on cardiac membranes.

**Correct Answer:** D. Hyperpolarization.