**Core Concept**
Morbid obesity is a severe form of obesity characterized by a significantly elevated body mass index (BMI), which has serious implications for an individual's health and quality of life. The World Health Organization (WHO) defines morbid obesity as a BMI of 40 or higher.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's definition of morbid obesity. A BMI of 40 or higher is associated with a significant increase in the risk of developing obesity-related comorbidities, such as type 2 diabetes, hypertension, and cardiovascular disease. This is because excess body fat, particularly visceral fat, leads to chronic inflammation, insulin resistance, and metabolic dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** A BMI of 30-34.9 is classified as class I obesity, not morbid obesity.
**Option B:** A BMI of 35-39.9 is classified as class II obesity, not morbid obesity.
**Option C:** A BMI of 25-29.9 is classified as overweight, not morbid obesity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that morbid obesity is a chronic condition that requires comprehensive management, including lifestyle modifications, pharmacotherapy, and bariatric surgery in severe cases. Early intervention and multidisciplinary care can improve health outcomes and reduce the risk of obesity-related comorbidities.

**Correct Answer: D. A BMI of 40 or higher.**