**Core Concept**
The toxic principle of certain plants refers to the bioactive compounds responsible for their adverse effects on humans. These compounds can be alkaloids, glycosides, or other secondary metabolites that interact with biological systems to produce toxicity.

**Why the Correct Answer is Right**
The toxic principle of the plant in question is a glycoside called digoxin, a cardiac glycoside extracted from the foxglove plant (Digitalis purpurea). Digoxin works by inhibiting the sodium-potassium ATPase pump in cardiac cells, leading to increased intracellular sodium and calcium levels. This results in enhanced contractility and increased heart rate. Digoxin also has a vagolytic effect, which can increase heart rate by blocking the action of acetylcholine on the sinoatrial node.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct toxic principle. The foxglove plant contains several cardiac glycosides, but digoxin is the most well-known and clinically relevant one.
**Option B:** This option is incorrect because it is not a known toxic principle of the foxglove plant. While the plant contains several bioactive compounds, digoxin is the primary toxic agent.
**Option C:** This option is incorrect because it is not a correct description of the toxic principle of the foxglove plant. The plant's toxic effects are primarily due to digoxin, not a combination of multiple compounds.

**Clinical Pearl / High-Yield Fact**
The foxglove plant has been used in medicine for centuries, and its cardiac glycosides have revolutionized the treatment of heart failure. However, digoxin toxicity is a significant concern, particularly in patients with renal impairment or electrolyte imbalances.

**Correct Answer: D. Digoxin.**