**Core Concept**
The concept of obesity being defined as being overweight by a specific percentage above accepted standards refers to the body mass index (BMI) classification system. BMI is a widely used measurement to categorize individuals into different weight categories, including underweight, normal weight, overweight, and obese.

**Why the Correct Answer is Right**
According to the World Health Organization (WHO) and other global health authorities, a person is considered obese if their BMI is 30 or more, which is equivalent to being overweight by 30% or more above the accepted standards. This classification system helps healthcare providers identify individuals who are at a higher risk of developing obesity-related health problems, such as diabetes, cardiovascular disease, and certain types of cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no commonly accepted definition of obesity as being overweight by 20% above accepted standards. This option may be a distractor, but it is not a recognized classification system.
**Option B:** Being overweight by 25% above accepted standards does not meet the criteria for obesity as defined by global health authorities.
**Option C:** There is no widely accepted definition of obesity as being overweight by 40% above accepted standards.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that BMI is a simple calculation that can be used to estimate an individual's body fat percentage, but it has limitations. For example, BMI may not accurately reflect body fat distribution in individuals with a muscular build or in those from certain ethnic backgrounds.

**Correct Answer: D. 30%**