**Core Concept**
The question focuses on non-sedating antiallergic drugs, which are typically classified as histamine receptor antagonists or leukotriene modifiers. These medications are designed to alleviate allergy symptoms without causing drowsiness.

**Why the Correct Answer is Right**
Non-sedating antiallergic drugs work by blocking histamine receptors or inhibiting the production of leukotrienes. Examples include loratadine, cetirizine, and fexofenadine, which are all histamine receptor antagonists. These medications are preferred over sedating antihistamines like diphenhydramine.

**Why Each Wrong Option is Incorrect**
**Option A:** Loratadine is a non-sedating antihistamine, making it a correct choice for non-sedating antiallergic drugs.
**Option C:** Fexofenadine is also a non-sedating antihistamine and is used in the treatment of allergic rhinitis and urticaria.

**Clinical Pearl / High-Yield Fact**
When choosing between different antihistamines, it's essential to consider whether sedation is a side effect that needs to be avoided. Non-sedating antihistamines like loratadine and fexofenadine are often preferred in patients who need to remain alert during the day.

**Correct Answer: D. Diphenhydramine is a sedating antihistamine.**