**Core Concept**
The question tests the understanding of **beta blockers** and their stereoisomerism, specifically which **isomer** is responsible for the **beta-blocking action**. This requires knowledge of **pharmacology** and **stereoselectivity** in drug action.

**Why the Correct Answer is Right**
The correct answer is related to the fact that the **d-isomer** of a particular beta blocker is responsible for its therapeutic effect. This is because the **d-isomer** has a higher **affinity** for **beta-adrenergic receptors**, leading to effective **blockade** of these receptors and resulting in the desired **therapeutic effect**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the isomer responsible for the beta-blocking action.
**Option B:** Incorrect as it is not the correct beta blocker with the d-isomer responsible for the action.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **propranolol** is an example where the **d-isomer** is more potent than the **l-isomer** in blocking **beta-adrenergic receptors**, making it a key point for exams.

**Correct Answer:** D. Propranolol