**Core Concept**
The World Health Organization (WHO) defines adolescence as a critical period of growth and development, spanning from the onset of puberty to the transition to adulthood. This stage is characterized by significant physical, emotional, and social changes.

**Why the Correct Answer is Right**
According to the WHO, adolescence is defined as the period from 10 to 19 years of age. This age range is based on the understanding that puberty typically begins around 10 years of age and continues until approximately 19 years of age. During this period, adolescents experience rapid growth, brain development, and the onset of reproductive capabilities. The WHO's definition of adolescence is widely accepted and used as a basis for healthcare and education policies.

**Why Each Wrong Option is Incorrect**
**Option B:** This age range is too narrow, as it only spans from 10 to 14 years of age, and does not account for the extended period of adolescence that continues beyond 14 years.
**Option C:** This age range is too broad, as it includes young adulthood and does not accurately reflect the specific developmental needs and challenges of adolescence.
**Option D:** This age range is incomplete, as it starts at 9 years of age, which is before the typical onset of puberty, and only goes up to 14 years of age.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that adolescence is a critical period of vulnerability, with increased risks of mental health issues, substance abuse, and injury. Healthcare providers should be aware of these risks and provide targeted support and education to adolescents and their families.

**✓ Correct Answer: A. 10-19 years**