## Core Concept
The pulse polio immunization (PPI) campaign targets children under the age of 5 years to immunize them against poliomyelitis. This campaign is crucial in areas where polio is endemic or has a high risk of outbreaks. The goal is to boost immunity in this vulnerable age group.

## Why the Correct Answer is Right
The correct answer focuses on children under 5 years because this age group is most susceptible to poliovirus infection and its complications, such as poliomyelitis. Poliovirus primarily affects children under 5 years, and they are at a higher risk of developing severe forms of the disease. Immunizing this age group helps in achieving herd immunity and preventing the spread of the virus.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately represent the target group for pulse polio immunization.
- **Option B:** Similarly, this option does not correctly identify the target group for PPI.
- **Option D:** This option is also incorrect as it does not specify children under 5 years as the target group.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the pulse polio immunization campaign primarily targets children under 5 years. This is a high-yield fact because it directly relates to public health strategies for polio eradication. Understanding the target population for such campaigns is crucial for both medical professionals and the general public to ensure effective participation and support for these initiatives.

## Correct Answer: B. Under 5 years.