## Core Concept
The **Body Mass Index (BMI)** is a widely used tool to categorize individuals into various weight categories, including underweight, normal weight, overweight, and obesity. BMI is calculated by dividing a person's weight in kilograms by their height in meters squared (kg/m^2). The classification of obesity based on BMI is crucial for assessing health risks associated with excess body fat.

## Why the Correct Answer is Right
The World Health Organization (WHO) defines the following BMI categories: underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obesity (≥30). The classification of obesity starts at a BMI of **30 or more**, which corresponds to option . This cutoff is significant because a BMI of 30 or higher is associated with an increased risk of various health conditions, including diabetes mellitus, cardiovascular diseases, and certain types of cancer. ## Why Each Wrong Option is Incorrect - **Option A:** 40 is incorrect because, although a BMI of greater than 40 indicates **class III obesity** (also known as severe, extreme, or morbid obesity), the threshold for classifying obesity starts at 30.

## Clinical Pearl / High-Yield Fact
A key point to remember is that BMI is not a perfect measure, as it does not directly assess body fatness or distinguish between lean body mass and body fat. However, for clinical and population health purposes, BMI remains a useful and practical tool for identifying individuals at increased risk of health problems due to excess body fat. A BMI of **30 or more** is a critical threshold for increased health risks.

## Correct Answer: .