**Core Concept:** Adolescence is a specific age range defined by World Health Organization (WHO) for developmental stages of human beings.

**Why the Correct Answer is Right:** The correct answer refers to the WHO's definition of adolescence, which is a period of human development that spans from approximately 10 to 19 years of age. In this context, adolescence is marked by significant physiological, psychological, and social changes that occur during growth and development.

**Why Each Wrong Option is Incorrect:**
A. **Option A (14 years):** This answer is incorrect because it does not align with the WHO definition of adolescence, which is a broader range from 10 to 19 years.
B. **Option B (20 years):** This answer is wrong due to the same reason, failing to align with the WHO's adolescence definition, which is a span from 10 to 19 years.
C. **Option C (18 years):** This answer is incorrect as it only considers one end of the adolescence age range (18 years), leaving out the crucial lower end (10 years) and the entire middle portion of the range.
D. **Option D (25 years):** This answer is wrong for the same reason as the other incorrect options, not aligning with the WHO's adolescence definition that spans from 10 to 19 years.

**Clinical Pearl / High-Yield Fact:** Understanding adolescence's WHO definition is crucial for healthcare professionals as it helps in providing appropriate interventions and guidance for this developmental stage.

**Correct Answer:** .