## Core Concept
The World Health Organization (WHO) defines various age groups for humans, and for adolescents, it specifies a particular range that is widely recognized in the field of public health and medicine. This definition is crucial for targeting health interventions, education, and policy-making.

## Why the Correct Answer is Right
The WHO defines the adolescent age group as **10-19 years**. This age range is significant because it encompasses the period of life when individuals undergo significant physical, psychological, and social changes as they transition from childhood to adulthood. During adolescence, individuals are particularly vulnerable to various health risks, and understanding this age group helps in designing appropriate health and educational programs.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately reflect the age range defined by WHO for adolescents.
- **Option B:** Similarly, this option does not match the WHO's definition for the adolescent age group.
- **Option D:** This option also does not correspond with the WHO's specified age range for adolescents.

## Clinical Pearl / High-Yield Fact
A crucial point to remember is that the adolescent age group, as defined by WHO, is **10-19 years**. This is a critical period for intervention in health and education to prevent long-term consequences on physical and mental well-being.

## Correct Answer: B. 10-19 years.