**Core Concept**
Digoxin is a cardiac glycoside used in the management of heart failure and atrial fibrillation. It increases the force of cardiac contractions and slows the heart rate by inhibiting the sodium-potassium ATPase pump, leading to an increase in intracellular calcium and subsequent contraction.

**Why the Correct Answer is Right**
Digoxin works by competitively inhibiting the sodium-potassium ATPase pump in cardiac cells, which increases the intracellular sodium concentration. This increase in intracellular sodium leads to an increase in intracellular calcium through the sodium-calcium exchange mechanism, resulting in enhanced cardiac contractility. The inhibition of the sodium-potassium ATPase pump also decreases the rate of contraction by prolonging the action potential duration.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because digoxin does not primarily act by increasing the heart rate; in fact, it can slow the heart rate in certain situations.
**Option B:** Incorrect because digoxin's primary mechanism is not through the inhibition of the calcium channel; it actually increases intracellular calcium indirectly.
**Option C:** Incorrect because digoxin does not primarily act by increasing the potassium level; in fact, it can cause hyperkalemia as a side effect.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor potassium levels closely when administering digoxin, as the drug can cause hyperkalemia, especially in patients with renal impairment.

**Correct Answer: C. Digoxin primarily increases the force of cardiac contractions by increasing intracellular calcium through the sodium-potassium ATPase pump inhibition.**