Which of the following antidotes would be best for severe digitalis toxicity?

In cases of severe digitalis toxicity, the most effective antidote is Digoxin immune Fab (Digibind). This antidote specifically binds to digoxin, a cardiac glycoside, which is primarily responsible for the toxicity in patients who have ingested too much of the drug or who are experiencing adverse reactions to therapeutic doses.

The mechanism of action for Digoxin immune Fab involves the formation of a complex with the digoxin molecules, which effectively neutralizes their effects on cardiac tissue. This helps to restore normal heart function by reducing the bioavailability of digoxin in the bloodstream and consequently alleviating symptoms such as bradycardia, arrhythmias, and gastrointestinal disturbances associated with digitalis toxicity.

Other substances listed might not have a direct therapeutic role in addressing digitalis toxicity. For instance, flumazenil is primarily used as an antidote for benzodiazepine overdose, naloxone is used to reverse opioid toxicity, and pencillamine is a chelating agent primarily employed in the treatment of heavy metal poisoning and certain conditions like Wilson's disease.