**Core Concept**
Obesity is a medical condition characterized by excessive body fat, which can lead to various health complications. The World Health Organization (WHO) defines obesity based on the body mass index (BMI), which is calculated by dividing the person's weight in kilograms by the square of their height in meters.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's definition of obesity, which categorizes individuals with a BMI of 30 or higher as obese. This cutoff is used globally to identify individuals at risk of obesity-related health issues, such as diabetes, cardiovascular disease, and certain types of cancer. The BMI calculation takes into account the individual's weight and height, providing a standardized measure to assess their body fat percentage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a BMI of 25-29.9 is classified as overweight, not obese.
**Option B:** This option is incorrect because a BMI of 18.5-24.9 is classified as normal weight, not obese.
**Option C:** This option is incorrect because a BMI of 25 is classified as overweight, not obese.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that BMI is not a perfect measure, as it does not account for muscle mass or body composition. However, it remains a widely used and effective tool for screening and identifying individuals at risk of obesity-related health issues.

**Correct Answer:** . Correct Answer: C.