The mechanism of action of "Terfenadine" (Seldane) is:

First, let us recall a basic biological fact: histamine exerts many of its physiological effects by binding to specialized cell-surface proteins called histamine receptors, mainly of the $$H_1$$ type in the context of allergy.

Now, the class of drugs to which Terfenadine (brand name “Seldane”) belongs is known as “second-generation antihistamines.” The word “antihistamine” itself literally means “against histamine,” so, from the very name, we already suspect that the drug must somehow oppose the action of histamine rather than enhance it.

We have two major ways in which a substance could oppose histamine:

(i) It might stop histamine from being released in the first place, i.e. inhibit its secretion from mast cells.

(ii) It might allow histamine to be released but then block, or antagonize, the receptors so that histamine cannot bind and trigger its usual effects.

Terfenadine belongs to category (ii). Pharmacologically, it is described as an $$H_1$$-receptor antagonist, or more informally, an “$$H_1$$ blocker.” That is, it fits into the $$H_1$$ receptor’s binding site without activating the receptor, thus preventing endogenous histamine from attaching and initiating the cascade that produces itching, swelling, sneezing, and other allergic symptoms.

So, rather than “activating” the histamine receptor (which would mimic histamine) or “helping in secretion” or “inhibiting secretion,” Terfenadine actually inhibits the action of the histamine receptor itself. It neutralizes the receptor’s response, even though histamine molecules might still be floating around in the extracellular fluid.

Comparing this mechanistic description with the options provided, we see that option D—“Inhibits the action of histamine receptor”—matches perfectly.

Hence, the correct answer is Option 4.