**Core Concept**
The question tests the understanding of **dose-response relationships** and **selectivity** of drugs, specifically **beta-2 adrenergic agonists** like salbutamol. This concept is crucial in pharmacology as it relates to the therapeutic index of a drug.

**Why the Correct Answer is Right**
Salbutamol is a **beta-2 adrenergic agonist** that is primarily used for its **bronchodilatory effects** in conditions like asthma. The fact that its dose-response curves for bronchodilation and **tachycardia** (a common side effect due to beta-1 receptor activation) are widely separated suggests that salbutamol has a higher **selectivity** for beta-2 receptors over beta-1 receptors at lower doses, thus minimizing **cardiovascular side effects**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the separation of dose-response curves indicates selectivity, not non-selectivity.
**Option B:** Incorrect as the question does not provide information about the drug's efficacy in different patient populations.
**Option C:** Incorrect because while salbutamol does have therapeutic effects, the question specifically addresses the separation of dose-response curves for desired and undesired effects.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **beta-2 selectivity** is a key factor in the clinical use of salbutamol, allowing for effective **bronchodilation** with minimal **cardiac side effects** at therapeutic doses.

**Correct Answer:** D. selective for its desired effect.