**Core Concept**
The Na+-K+ ATPase is an enzyme that plays a crucial role in maintaining the electrolyte balance and membrane potential of cells by pumping sodium ions out and potassium ions into the cell against their concentration gradients. This process requires energy derived from the hydrolysis of ATP.

**Why the Correct Answer is Right**
K+ binds onto the E2-P state of the Na+-K+ ATPase, also known as the phosphorylated state. This binding causes a conformational change in the enzyme, allowing the K+ ions to be transported into the cell against their concentration gradient. The binding of K+ to the E2-P state is a critical step in the Na+-K+ ATPase's mechanism, enabling the pump to function effectively.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the E2 state, also known as the dephosphorylated state, is the conformational state where Na+ binds, not K+.
**Option B:** Incorrect because the binding of K+ to the E2-P state is specific and distinct from the binding of Na+ to the E2 state.
**Option C:** Incorrect because this option does not accurately describe the step where K+ binds onto the Na+-K+ ATPase.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Na+-K+ ATPase is an electrogenic pump, meaning it can generate a membrane potential due to the unequal transport of ions across the membrane.

**Correct Answer: D.**