**Core Concept**
The **Body Mass Index (BMI)** is a widely used measurement to categorize individuals into various weight categories, including underweight, normal weight, overweight, and obese. BMI is calculated by dividing the person's weight in kilograms by the square of their height in meters.

**Why the Correct Answer is Right**
A person is considered **obese** if their BMI is 30 or higher. This is because a BMI of 30 or above is associated with a significantly increased risk of developing **obesity-related health problems**, such as **diabetes**, **hypertension**, and **heart disease**. The World Health Organization (WHO) uses the following BMI categories: underweight (BMI < 18.5), normal weight (BMI = 18.5-24.9), overweight (BMI = 25-29.9), and obese (BMI ≥ 30). **Why Each Wrong Option is Incorrect** **Option A:** A BMI of less than 18.5 is considered underweight, not obese. **Option B:** A BMI of 18.5-24.9 is considered normal weight, not obese. **Option C:** A BMI of 25-29.9 is considered overweight, not obese. **Clinical Pearl / High-Yield Fact** Remember that BMI is not a perfect measure, as it does not account for **muscle mass** or **body composition**. However, it is a useful tool for identifying individuals who may be at risk for obesity-related health problems. **Correct Answer:** D. 30 or higher