**Core Concept**
The unit of ecological study refers to the smallest unit of analysis in epidemiology, which is used to examine the association between exposure and outcome at the population level. In other words, it is the basic building block of ecological studies. This concept is crucial in understanding the strengths and limitations of ecological studies.

**Why the Correct Answer is Right**
The correct answer is **cluster**. A cluster is a group of individuals who share a common characteristic, such as geographic location, occupation, or social network. In ecological studies, clusters are used to group individuals together and analyze the association between exposure and outcome at the population level. This approach is particularly useful in situations where individual-level data are not available or are difficult to collect. For example, in a study examining the relationship between air pollution and respiratory disease, clusters might be defined as neighborhoods or cities with varying levels of air pollution.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **individual** is the unit of analysis in observational studies, not ecological studies.
**Option B:** **Society** is too broad a term and is not a specific unit of analysis in ecological studies.
**Option C:** **Community** is a related concept, but it is not the specific unit of analysis used in ecological studies.

**Clinical Pearl / High-Yield Fact**
When designing an ecological study, it is essential to define the unit of analysis carefully to avoid biases and ensure that the results are generalizable to the population of interest.

**Correct Answer:** C. Cluster