**Core Concept**
The question requires knowledge of antiarrhythmic drug therapy for treating ventricular extrasystoles caused by digoxin toxicity. Digoxin toxicity can lead to atrioventricular (AV) node blockade, increased automaticity, and ventricular arrhythmias.

**Why the Correct Answer is Right**
Digoxin toxicity-induced ventricular extrasystoles are best treated with a beta-blocker. Beta-blockers, specifically **Atenolol** (a non-selective beta-blocker), counteract the increased automaticity and enhance the refractoriness of the myocardium. This action is crucial in preventing the propagation of ventricular arrhythmias. By blocking beta-1 receptors in the heart, atenolol reduces the effects of digoxin toxicity on the myocardium, thereby stabilizing the heart rate and rhythm.

**Why Each Wrong Option is Incorrect**
**Option A:** Lignocaine (Lidocaine) is an antiarrhythmic drug that is effective in treating ventricular arrhythmias but is not the first choice for digoxin toxicity-induced arrhythmias. It is more suitable for wide QRS complex tachyarrhythmias.

**Option B:** Phenytoin is an antiepileptic and antiarrhythmic drug, but it is not indicated for treating ventricular extrasystoles due to digoxin toxicity. It can actually worsen digoxin toxicity by prolonging the QT interval.

**Option C:** Amiodarone is a powerful antiarrhythmic medication with a broad spectrum of activity, but it is not the first choice for treating ventricular extrasystoles due to digoxin toxicity. Its use is generally reserved for more severe arrhythmias or when other treatments have failed.

**Clinical Pearl / High-Yield Fact**
When treating digoxin toxicity, it is essential to remember that beta-blockers like atenolol can mask the signs of toxicity, such as nausea and vomiting, by reducing the heart rate. Therefore, it is crucial to monitor digoxin levels and cardiac function closely when using beta-blockers for this indication.

**Correct Answer:** A. Atenolol