**Core Concept**
Digoxin is a cardiac glycoside used to treat heart failure and atrial fibrillation. The therapeutic plasma level of digoxin is crucial for its efficacy and safety, as it can cause toxicity if levels become too high.

**Why the Correct Answer is Right**
The therapeutic plasma level of digoxin is between 0.8-1.2 ng/mL. This range is considered optimal for maintaining the drug's efficacy while minimizing the risk of toxicity. Digoxin works by inhibiting the sodium-potassium ATPase pump, leading to increased intracellular sodium and calcium levels, which in turn increases the contractility of the heart muscle. The drug's plasma levels are tightly regulated by the kidneys, which excrete the drug through the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because digoxin levels below 0.8 ng/mL may not be sufficient to achieve the desired therapeutic effect.
* **Option B:** This option is incorrect because digoxin levels above 1.2 ng/mL increase the risk of toxicity, including arrhythmias and heart block.
* **Option C:** This option is incorrect because it is not a valid therapeutic plasma level range for digoxin.
* **Option D:** This option is incorrect because it is not a valid therapeutic plasma level range for digoxin.

**Clinical Pearl / High-Yield Fact**
To avoid digoxin toxicity, it is essential to monitor plasma levels regularly, especially in patients with renal impairment or those taking other medications that interact with digoxin.

**Correct Answer: B. 1.2 ng/mL**