**Core Concept**
The question is testing the understanding of epidemiological study designs, specifically the type of study that involves long-term observation of a group of individuals to determine the incidence of a particular disease or outcome.

**Why the Correct Answer is Right**
This study is an example of a **cohort study**. In a cohort study, a group of individuals who share a common characteristic (in this case, smoking) are followed over time to observe the development of a specific outcome (lung cancer). The study aims to establish a cause-and-effect relationship between the exposure (smoking) and the outcome (lung cancer). The 10-year follow-up period allows researchers to capture the incidence of lung cancer in the cohort, providing valuable data on the risk factors associated with this disease.

**Why Each Wrong Option is Incorrect**
**Option A:** A case-control study is incorrect because it involves comparing individuals with a specific disease (lung cancer) to those without the disease, looking back in time to identify potential risk factors. This study does not involve long-term observation of a cohort.
**Option B:** An experimental study is incorrect because it involves manipulating the exposure (e.g., smoking) to observe its effect on the outcome (e.g., lung cancer). This study is observational, not experimental.
**Option C:** A cross-sectional study is incorrect because it involves a snapshot of a population at a single point in time, rather than observing a cohort over a prolonged period.

**Clinical Pearl / High-Yield Fact**
When designing observational studies, researchers must consider the potential for confounding variables, such as age, sex, and socioeconomic status, which can affect the outcome and bias the results. Adjusting for these variables through statistical analysis can help control for confounding and increase the validity of the study findings.

**Correct Answer: C. Cohort study.**