## Core Concept
This question tests the understanding of epidemiological study designs, specifically sampling methods. The described method involves selecting every nth person from a population, which is a systematic approach to sampling. This technique helps ensure that the sample is representative of the population.

## Why the Correct Answer is Right
The correct answer, **C. Systematic sampling**, is right because systematic sampling involves selecting samples based on a fixed interval or system (e.g., every 10th item). This method starts with a random pick from the first n items, and then every nth item is selected. For instance, if the population list is used and every 10th person is chosen, it qualifies as systematic sampling. This method reduces bias and allows for a spread across the population.

## Why Each Wrong Option is Incorrect
- **Option A: Simple Random Sampling** is incorrect because it involves selecting participants randomly from the population, where every individual has an equal chance of being chosen, without any specific interval or system.
- **Option B: Stratified Sampling** is incorrect because it involves dividing the population into distinct subgroups (strata) and then performing simple random sampling within each subgroup. This ensures that each subgroup is represented in the sample.
- **Option D: Cluster Sampling** is incorrect because it involves dividing the population into clusters (e.g., geographic areas) and then randomly selecting some of these clusters to be included in the study. All or a random sample of individuals within these clusters may be studied.

## Clinical Pearl / High-Yield Fact
A key point to remember is that systematic sampling is quicker and more practical for large populations compared to simple random sampling. However, it might introduce bias if there's a pattern in the population list that correlates with the study's variables.

## Correct Answer: C. Systematic sampling.