## Core Concept
Cluster sampling is a method used in statistics where the population is divided into clusters, and a random selection of these clusters is chosen for the sample. In the context of evaluating the coverage of a national immunization programme, cluster sampling can be efficiently used to assess the immunization status of children across different regions. This method is particularly useful for large-scale, geographically spread populations.

## Why the Correct Answer is Right
The correct answer, **30 clusters**, refers to the World Health Organization's (WHO) Expanded Programme on Immunization (EPI) cluster survey method. This method involves selecting 30 clusters from a defined area to estimate vaccination coverage. The process typically involves a two-stage sampling technique where 30 clusters are randomly selected from a list of clusters in the area (often districts or villages), and then within each cluster, a certain number of households or children are sampled to gather data on immunization status. This technique provides a balance between precision and practicality for field surveys.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because there is no widely recognized standard for 5 clusters in the context of evaluating immunization programme coverage.
- **Option B:** Similarly, **10 clusters** is not a standard number associated with a widely used method for assessing immunization coverage.
- **Option D:** **40 clusters** might seem plausible but is not the standard number used in the EPI cluster survey method.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the EPI cluster survey method uses **30 clusters** to assess immunization coverage. This method has become a standard tool for evaluating the effectiveness of immunization programs, especially in resource-limited settings. Understanding this method is crucial for public health professionals and researchers conducting field surveys.

## Correct Answer: C. 30 clusters.