## Core Concept
The National Immunization Programme (NIP) in India aims to immunize children against various vaccine-preventable diseases. Cluster sampling is a method used for surveying populations, especially in areas where a census is not available or feasible. In cluster sampling, the population is divided into clusters, and a random selection of these clusters is chosen for the survey.

## Why the Correct Answer is Right
The correct answer, . 30, is related to the cluster sampling method used in the National Immunization Programme. Specifically, the World Health Organization (WHO) and other global health initiatives often use a method known as the "30 clusters" technique for vaccine efficacy and coverage evaluations. This involves dividing the area to be surveyed into smaller, manageable clusters, typically 30 in number, and then randomly selecting some of these clusters for detailed study. This method helps ensure a representative sample of the population.

## Why Each Wrong Option is Incorrect
- **Option A:** . 10 - This option suggests 10 clusters, which might not provide a sufficiently representative sample of the population for accurate immunization coverage assessment.
- **Option B:** . 20 - While 20 clusters could offer a better representation than 10, it's not the standard number used in many immunization surveys.
- **Option D:** . 40 - Although 40 clusters could provide an even more detailed picture, the standard and widely accepted number for such surveys is not 40.

## Clinical Pearl / High-Yield Fact
A key fact to remember is that the "30 clusters" technique is a well-established method for evaluating vaccine coverage and efficacy in various public health programs, including the National Immunization Programme. This technique helps in making informed decisions about vaccine distribution and immunization strategies.

## Correct Answer Line
**Correct Answer: C. 30.**