**Core Concept:** A second-generation antihistamine is a newer generation of antihistamines that have fewer sedative side effects compared to the first-generation antihistamines. They act on the central nervous system to inhibit histamine release and binding to H1 receptors.

**Why the Correct Answer is Right:**
Diphenhydramine (Option D) is a first-generation antihistamine, also known as a "sedating antihistamine." It acts on both central and peripheral H1 receptors, leading to sedation and other side effects.

**Why Each Wrong Option is Incorrect:**
A. Cetirizine (Option C) is a second-generation antihistamine that does not cross the blood-brain barrier, reducing sedation.
B. Loratadine (Option B) is also a second-generation antihistamine with minimal central nervous system effects.
C. Terfenadine (Option A) is another second-generation antihistamine that does not cross the blood-brain barrier, avoiding sedation.

**Clinical Pearl:**
Second-generation antihistamines are commonly used in the treatment of allergic symptoms like hay fever, with fewer side effects than first-generation antihistamines.

**Correct Answer:** Cetirizine (Option C) is the correct answer as it is a second-generation antihistamine that does not cross the blood-brain barrier, reducing sedation.