## Core Concept
The question assesses knowledge of various sampling techniques used in research and epidemiology. These techniques include snowball sampling, systematic random sampling, stratified random sampling, and cluster sampling. Each method has distinct applications, advantages, and limitations.

## Why the Correct Answer is Right
- **Snowball sampling** is indeed used for hard-to-reach or hidden populations, where existing study subjects recruit future subjects from among their acquaintances. So, statement (a) is correct.
- **Stratified random sampling** involves dividing the population into distinct subgroups or strata, which are then sampled. This ensures that each subgroup is represented in the sample. So, statement (c) is correct.
- Statement (b) about more sample in systematic random sampling is not necessarily true as a defining characteristic. Systematic random sampling involves selecting samples based on a fixed interval (e.g., every 10th item), which doesn't inherently mean it yields more samples.
- **Cluster sampling** involves selecting clusters or groups rather than individual subjects. It's often used for logistical and cost reasons but can be less precise. Statement (d) says cluster sampling is less cost-effective, which can be misleading. Cluster sampling is often more cost-effective, especially for large, dispersed populations.

## Why Each Wrong Option is Incorrect
- **Option B:** This option suggests that only statements (a) and (c) are correct. However, without evaluating the truthfulness of all statements, we cannot immediately confirm or deny this.
- **Option D:** Suggests that only (a) and (d) are correct. Given the potential inaccuracies in statement (d) regarding cost-effectiveness, this might not hold.
- The incorrectness hinges on the evaluation of each statement's accuracy.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **stratified sampling** ensures representation of subgroups within a population, which is crucial for comparative studies. **Snowball sampling** is particularly useful for studying sensitive topics or populations that are hard to reach through conventional methods.

## Correct Answer: C. a, c.