**Core Concept**
The question is testing the student's knowledge of the anticholinergic activity of different H1 blockers, also known as antihistamines. H1 blockers are a class of medications used to treat allergic reactions by blocking the action of histamine at the H1 receptor. However, some H1 blockers can also exhibit anticholinergic activity, which is the inhibition of the action of acetylcholine at muscarinic receptors, leading to various side effects such as dry mouth, constipation, and urinary retention.

**Why the Correct Answer is Right**
Chlorpheniramine is a first-generation H1 blocker that is known for its high anticholinergic activity. This is because it can cross the blood-brain barrier and bind to muscarinic receptors in the central nervous system, leading to a range of side effects. Chlorpheniramine is often used in combination with other medications to treat conditions such as allergic rhinitis, but its high anticholinergic activity can limit its use in certain patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Cetirizine is a second-generation H1 blocker that is known for its low anticholinergic activity and lack of sedation. It is commonly used to treat allergic reactions and is often preferred over first-generation H1 blockers due to its improved safety profile.
**Option C:** Fexofenadine is another second-generation H1 blocker that is known for its low anticholinergic activity and is often used to treat allergic reactions. It is also less likely to cause sedation compared to first-generation H1 blockers.
**Option D:** Astemizole is a second-generation H1 blocker that was withdrawn from the market due to concerns over its potential to cause cardiac arrhythmias. It is not known for its anticholinergic activity.

**Clinical Pearl / High-Yield Fact**
When choosing an H1 blocker, it is essential to consider the patient's age, medical history, and potential for anticholinergic side effects. First-generation H1 blockers like chlorpheniramine are generally avoided in elderly patients due to their high anticholinergic activity, which can increase the risk of cognitive impairment and other adverse effects.

**✓ Correct Answer: B. Chlorpheniramine**