**Core Concept**
Cluster sampling is a method used in epidemiology and public health to assess the immunization status of children under an immunization program. This method involves dividing the population into clusters, which are then randomly sampled to estimate the prevalence of a particular disease or health indicator.

**Why the Correct Answer is Right**
Cluster sampling is particularly useful in immunization programs because it allows for the efficient assessment of a large population with limited resources. By selecting a representative sample of clusters, researchers can estimate the immunization coverage of the entire population. This method is also useful for identifying areas with low immunization coverage, which can inform targeted interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid sampling method.
**Option B:** This option is not relevant to assessing immunization status.
**Option C:** This option is a different sampling method used in epidemiology, but not specifically for assessing immunization status.

**Clinical Pearl / High-Yield Fact**
Cluster sampling is a cost-effective method for assessing immunization coverage, but it requires careful selection of clusters to ensure representativeness of the population.

**Correct Answer:** C. Cluster sampling.