**Core Concept**
Corpulence index is a numerical value that reflects an individual's degree of obesity, calculated based on their height and weight measurements. It is a simple yet effective tool used in clinical practice and public health settings to assess the prevalence of obesity within a population. The corpulence index is often used in conjunction with other anthropometric measures to monitor trends in obesity over time.

**Why the Correct Answer is Right**
The corpulence index is calculated using various formulas, but the most common one is the Quetelet's index, also known as the body mass index (BMI). BMI is calculated by dividing an individual's weight in kilograms by their height in meters squared (kg/m^2). A BMI of 30 or higher is typically considered obese, while a BMI between 25 and 29.9 is considered overweight. The corpulence index is a useful tool for identifying individuals at risk of obesity-related complications, such as diabetes, cardiovascular disease, and certain types of cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Copper level in the serum - The corpulence index has no relation to copper levels in the blood, which are used to diagnose conditions such as Wilson's disease.
**Option B:** Iron loss in faeces - The corpulence index is not a measure of iron loss or any other nutrient deficiency.
**Option D:** Income inequality - The corpulence index is a physiological measure, not an economic one, and is not related to income inequality.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) defines obesity as a BMI of 30 or higher, which is equivalent to a corpulence index of 30 or higher. This classification system is widely used in clinical practice and public health settings to identify individuals at risk of obesity-related complications.

✓ Correct Answer: C. Obesity