**Core Concept**
Salbutamol is a short-acting β2 adrenergic receptor agonist (SABA) used for the relief of bronchospasm in conditions such as asthma and COPD. Its mechanism of action involves binding to β2 receptors in the smooth muscle of the bronchi, leading to relaxation and bronchodilation.

**Why the Correct Answer is Right**
The most prominent and dose-related side effect of salbutamol is tachycardia. This occurs due to the non-selective stimulation of β2 and β1 adrenergic receptors, leading to increased heart rate and contractility. The β2 selective action of salbutamol is dose-dependent, and at higher doses, the non-selective effects on β1 receptors become more pronounced, resulting in tachycardia.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypokalemia is a potential side effect of salbutamol due to its potassium-wasting effect, but it is not the most prominent and dose-related side effect.

**Option B:** Tremors can occur with salbutamol due to its stimulating effects on the nervous system, but they are not as dose-related as tachycardia.

**Option C:** Headache can occur with salbutamol, but it is not a prominent or dose-related side effect compared to tachycardia.

**Clinical Pearl / High-Yield Fact**
Salbutamol can cause a paradoxical bronchospasm in some patients, especially those with severe asthma, highlighting the importance of careful patient selection and monitoring.

**Correct Answer:** C. Tachycardia