**Core Concept**
Sampling methods in research studies aim to accurately represent the population from which the sample is drawn. Natural groups refer to existing subgroups within a population that are already defined, such as by age, sex, or geographic location. The correct sampling method involves randomly selecting some of these pre-existing natural groups to ensure representation.

**Why the Correct Answer is Right**
Cluster sampling involves randomly selecting some of the pre-existing natural groups, also known as clusters, and then sampling from those selected clusters. This method is particularly useful when the population is dispersed or difficult to access. By selecting clusters, researchers can reduce the time and resources required for data collection while still achieving a representative sample. This approach is also useful in situations where sampling from the entire population is not feasible or cost-effective.

**Why Each Wrong Option is Incorrect**
**Option A:** Stratified sampling involves dividing the population into distinct subgroups or strata and then sampling from each subgroup separately. This method is used to ensure that each subgroup is represented in the sample, but it does not involve randomly selecting pre-existing natural groups.
**Option B:** Systematic sampling involves selecting samples based on a fixed interval or system, such as every 10th participant. This method is useful for ensuring that the sample is representative of the population, but it does not involve randomly selecting pre-existing natural groups.
**Option C:** Convenience sampling involves selecting samples based on ease of access or convenience, rather than randomly selecting pre-existing natural groups. This method is often used in pilot studies or when time is limited, but it can lead to biased samples.

**Clinical Pearl / High-Yield Fact**
When designing a research study, it's essential to carefully consider the sampling method to ensure that the sample is representative of the population. Cluster sampling can be a useful approach when working with large or dispersed populations, but it requires careful planning to avoid biases.

**Correct Answer:** C.