Step 1: Understand what an expectorant does.
An expectorant is a drug that helps loosen and thin mucus in the airways, making it easier to cough up.
Step 2: Recall the crude drugs and their main actions.
Digitalis is used as a cardiac glycoside for heart failure. Belladonna contains atropine and is used as an anticholinergic. Senna is a laxative used for constipation.
Step 3: Identify Liquorice and its uses.
Liquorice (from the root of Glycyrrhiza glabra) contains glycyrrhizin and other saponins. Saponins reduce surface tension in the airways and stimulate bronchial secretion, producing an expectorant effect.
Step 4: Why the distractors are wrong.
Digitalis acts on the heart (cardiac glycoside), not the respiratory tract. Belladonna dries secretions (anticholinergic), which is the opposite of expectorant action. Senna works in the gut as a stimulant laxative.
Step 5: Conclude.
Only Liquorice matches the definition of a crude drug expectorant through its saponin content.
Answer: Option (1) — Liquorice