**Core Concept**
The study aims to investigate the association between chronic obstructive pulmonary disease (COPD) and smoking, using data from government hospital records and cigarette sales records from finance and taxation departments. This study design is used to establish cause-and-effect relationships or associations between exposure and outcome variables.

**Why the Correct Answer is Right**
The correct answer is **C. Ecological study** because it involves analyzing data at the population or group level, rather than individual level. In this case, the study is examining the relationship between COPD incidence and cigarette sales data at the population level. Ecological studies are useful for identifying trends and associations but are limited by the ecological fallacy, where individual-level observations are inferred from group-level data.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not correct because a case-control study would involve comparing patients with COPD to those without the disease, but it would not use population-level data like cigarette sales records.
**Option B:** A randomized controlled trial (RCT) is not applicable here as it involves manipulating the exposure (smoking) and measuring the outcome (COPD) in a controlled setting, which is not feasible in this scenario.
**Option D:** A cohort study would involve following a group of people over time to examine the development of COPD in relation to smoking exposure, but it would not use population-level data like cigarette sales records.

**Clinical Pearl / High-Yield Fact**
Ecological studies are useful for identifying associations between exposure and outcome variables at the population level, but they are limited by the ecological fallacy, where individual-level observations are inferred from group-level data.

**Correct Answer: C. Ecological study**