**Core Concept**
The question is testing the definition of obesity based on Body Mass Index (BMI). BMI is a widely used measurement to categorize individuals as underweight, normal weight, overweight, or obese. The World Health Organization (WHO) has established BMI cut-offs for obesity, which are used globally for health surveillance and health promotion.

**Why the Correct Answer is Right**
The correct BMI cut-off for obesity is 30 kg/m^2 or higher. This is because a BMI of 30 or higher is associated with an increased risk of various health problems, including diabetes, cardiovascular disease, and certain types of cancer. The WHO defines obesity as a BMI of 30-39.9 as class 1 obesity, 40-49.9 as class 2 obesity, and 50 or higher as class 3 obesity. The mechanism behind the increased risk is the excess body fat, which leads to chronic inflammation, insulin resistance, and other metabolic changes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low to be considered obese. A BMI of 25-29.9 is classified as overweight, not obese.
**Option B:** This option is incorrect because it is too high to be considered the upper limit of obesity. While a BMI of 35 or higher is associated with a higher risk of health problems, it is not the upper limit of obesity.
**Option C:** This option is incorrect because it is not a valid BMI cut-off for obesity. The WHO has established specific BMI cut-offs for obesity, and this option does not match any of them.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that BMI is only an estimate of body fatness and may not accurately reflect it in certain populations, such as athletes or individuals with a muscular build. However, it remains a useful tool for health professionals to identify individuals at risk of obesity-related health problems.

**Correct Answer: C. 30 kg/m^2 or higher**