**Cluster Sampling**

**Core Concept**
Cluster sampling is a type of probability sampling method where the population is divided into clusters, and a random sample of these clusters is selected to be included in the study. The goal is to obtain a representative sample of the population by selecting a subset of clusters that are representative of the larger population.

**Why the Correct Answer is Right**
Cluster sampling is a useful method when the population is geographically dispersed or when the clusters are naturally occurring groups, such as schools or hospitals. The selected clusters are then sampled randomly, and all members of the selected clusters are included in the study. This approach can be more efficient and cost-effective than other sampling methods, such as simple random sampling.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because cluster sampling typically involves selecting a random sample of clusters, not a random sample of individuals within the clusters.
**Option B:** Incorrect because cluster sampling can be used in various settings, including geographic areas, schools, or hospitals, making it a versatile method.
**Option C:** Incorrect because cluster sampling can be used to study populations with complex structures, such as those with multiple subgroups or hierarchies.

**Clinical Pearl / High-Yield Fact**
When designing a study using cluster sampling, it's essential to ensure that the selected clusters are representative of the larger population and that the sample size is adequate to achieve the desired level of precision.

**Correct Answer: A.**