**Core Concept**
Digitalis, a cardiac glycoside, primarily exerts its positive inotropic effect by inhibiting the sodium-potassium ATPase pump in cardiac myocytes, leading to an increase in intracellular sodium and a subsequent increase in calcium levels through the sodium-calcium exchange mechanism.

**Why the Correct Answer is Right**
The inhibition of the sodium-potassium ATPase pump results in an increase in intracellular sodium, which in turn activates the sodium-calcium exchanger, causing an influx of calcium into the cardiac myocyte. This increase in intracellular calcium enhances the contractility of the heart muscle, thereby producing a positive inotropic effect. This effect is mediated through the inhibition of the sodium-potassium ATPase pump by the digitalis glycoside, digoxin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as digitalis does not have a significant effect on the L-type calcium channels directly. While calcium channels are involved in the mechanism, it is not the primary site of action for digitalis.
**Option B:** This option is incorrect as the beta-adrenergic receptors are primarily involved in the regulation of heart rate and contractility through the sympathetic nervous system, rather than being a direct site of action for digitalis.
**Option C:** This option is incorrect as the muscarinic receptors are primarily involved in the parasympathetic regulation of heart rate, rather than being a direct site of action for digitalis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that digitalis toxicity can occur due to increased levels of digitalis glycosides in the body, which can be caused by various factors such as renal impairment, hypokalemia, and hypercalcemia.

**Correct Answer:** C.