## Core Concept
The World Health Organization (WHO) uses the Body Mass Index (BMI) as a crucial indicator for assessing nutritional status. BMI is calculated as weight in kilograms divided by the square of height in meters (kg/m^2). It categorizes individuals into various groups such as underweight, normal weight, overweight, and obese.

## Why the Correct Answer is Right
The correct answer, , indicates that according to the WHO, a BMI of less than 18.5 is considered underweight or malnourished. This classification is critical for public health and clinical practice as it helps in identifying individuals at risk of malnutrition and its complications. The WHO's classification of BMI categories includes: underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (≥30). ## Why Each Wrong Option is Incorrect - **Option A:** This option is incorrect because a BMI of 18.5-24.9 is considered normal weight according to the WHO. - **Option B:** This option is incorrect because it does not accurately reflect the WHO's definition for underweight or malnourished individuals. - **Option D:** This option is incorrect because a BMI of ≥25 is considered overweight or obese, not underweight or malnourished. ## Clinical Pearl / High-Yield Fact A key point to remember is that a BMI of less than 18.5 is a critical threshold for identifying individuals who are underweight or at risk of malnutrition. This is particularly important in clinical settings for early intervention and prevention of malnutrition-related complications. ## Correct Answer Line **Correct Answer: C. <18.5.**