**Core Concept**
Obesity is a complex condition characterized by an excessive accumulation of body fat, which can lead to various health complications. The Body Mass Index (BMI) is a widely used measurement to assess weight status, and it is calculated by dividing an individual's weight in kilograms by the square of their height in meters.

**Why the Correct Answer is Right**
The World Health Organization (WHO) defines obesity as a BMI of 30 or higher. This classification is based on the fact that a BMI of 30 is associated with a significantly increased risk of obesity-related health problems, such as diabetes, cardiovascular disease, and certain types of cancer. **The WHO BMI classification system** is widely used globally to standardize the definition and diagnosis of obesity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a BMI of 25 is actually classified as overweight, not obese. A BMI of 25 indicates a higher risk of health problems compared to a normal BMI, but it does not meet the criteria for obesity.
**Option B:** This option is incorrect because a BMI of 18.5-24.9 is classified as normal, not obese. A normal BMI is associated with a lower risk of health problems compared to higher BMIs.
**Option C:** This option is incorrect because a BMI of 25-29.9 is classified as overweight, not obese. While this range indicates a higher risk of health problems, it does not meet the criteria for obesity.

**Clinical Pearl / High-Yield Fact**
When assessing a patient's weight status, it is essential to consider the limitations of BMI, as it does not take into account muscle mass or body composition. Therefore, BMI should be used in conjunction with other measurements, such as waist circumference and body fat percentage, to get a more accurate picture of an individual's health risk.

**Correct Answer:** D.