**Core Concept**
An ecological study examines aggregate data at the population level rather than individual-level outcomes. It compares exposure levels (e.g., drug use) with disease rates (e.g., asthma deaths) across different groups or regions, without tracking individuals.

**Why the Correct Answer is Right**
This scenario compares asthma death rates to anti-asthma drug sales at a population level in the UK, using group-level data (e.g., national statistics). It does not follow individual patients or assess causality through intervention. The study observes correlations between population-level drug sales and mortality rates, which is the hallmark of an ecological study.

**Why Each Wrong Option is Incorrect**
Option A: A cohort study follows individuals over time to assess disease development and exposures. This study does not track individuals or their asthma history.
Option B: A case-reference study (likely meant to be "case-control") compares individuals with and without a disease, which is not done here.
Option D: An experimental study involves active intervention and control groups, which is absent in this observational, population-level analysis.

**Clinical Pearl / High-Yield Fact**
Ecological studies are prone to ecological fallacy—inferring individual-level causality from group-level data. Always remember: correlation ≠ causation, especially in population-level data.

✓ Correct Answer: C. Ecological study.