## Core Concept
Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, normal weight, overweight, and obesity in adults. It is calculated by dividing an individual's weight in kilograms by the square of their height in meters (kg/m^2). This measurement is used worldwide to provide a straightforward and universally applicable method for assessing weight status.

## Why the Correct Answer is Right
The correct formula for BMI is weight in kilograms divided by the square of height in meters (kg/m^2). This means the correct completion of the sentence is "square of height". BMI is a widely used tool because it correlates well with body fat mass and has been shown to be a good indicator of health risks associated with excess body fat. The calculation does not require any complex equipment, making it accessible for use in a variety of settings.

## Why Each Wrong Option is Incorrect
- **Option A:** "height" - This option is incorrect because BMI is not simply a ratio of body weight to height, but specifically to the square of height. Using just height would not accurately reflect the relationship between weight and body size.
- **Option B:** "cube of height" - This option is incorrect because the established and widely accepted formula for BMI involves the square, not the cube, of height.
- **Option D:** "square root of height" - This option is also incorrect as it inaccurately represents the BMI formula; the correct calculation involves squaring, not taking the square root of, the height.

## Clinical Pearl / High-Yield Fact
A key point to remember is that BMI categories are defined as follows: underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (≥30). This classification can help in assessing the risk of various health conditions, such as diabetes, cardiovascular diseases, and certain types of cancer, that are associated with being underweight, overweight, or obese. ## Correct Answer: B. square of height.