**Core Concept**
The question is testing knowledge of digitalis-induced arrhythmias and the role of monovalent cations in reversing them. Digitalis, a cardiac glycoside, inhibits the Na+/K+-ATPase pump, leading to increased intracellular sodium and decreased intracellular potassium. This can cause arrhythmias, particularly in patients with underlying heart conditions. Reversing these arrhythmias often requires correcting the underlying electrolyte imbalance.

**Why the Correct Answer is Right**
The monovalent cation that can reverse a digitalis-induced arrhythmia is potassium (K+). Potassium helps to counteract the effects of digitalis on the Na+/K+-ATPase pump by increasing intracellular potassium levels. This can help to normalize the cardiac action potential and prevent arrhythmias. The administration of potassium can be life-saving in patients with severe digitalis toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because calcium (Ca2+) can actually exacerbate digitalis toxicity by increasing the contractility of the heart and making it more susceptible to arrhythmias.
* **Option B:** This option is incorrect because magnesium (Mg2+) can also worsen digitalis toxicity by further inhibiting the Na+/K+-ATPase pump and increasing intracellular sodium.
* **Option D:** This option is incorrect because sodium (Na+) is actually the monovalent cation that is increased in digitalis toxicity, not decreased.

**Clinical Pearl / High-Yield Fact**
In patients with digitalis toxicity, administering potassium can help to reverse arrhythmias, but it's essential to monitor potassium levels closely to avoid hyperkalemia.

**Correct Answer:** C. Potassium.