**Core Concept**
Quetelet's index is a measure of body mass index (BMI) that is used to assess an individual's weight status. This index is calculated by dividing the person's weight in kilograms by the square of their height in meters, providing a numerical value that can be used to categorize individuals as underweight, normal weight, overweight, or obese.

**Why the Correct Answer is Right**
Quetelet's index is calculated using the formula weight (in kg) / height (in meters)². This formula is based on the concept that body mass is directly proportional to the square of body height, allowing for a simple and effective way to categorize individuals based on their weight status. The resulting value is then used to determine the individual's BMI category, which is essential for assessing health risks associated with being underweight or overweight.

**Why Each Wrong Option is Incorrect**
**Option A:** This formula is incorrect as it does not take into account the square of the height, which is crucial for calculating the correct BMI value.
**Option B:** This formula is also incorrect as it does not use the correct units for weight and height, leading to an inaccurate calculation.
**Option C:** This formula is incorrect as it does not follow the correct mathematical operation of squaring the height, resulting in an incorrect BMI value.

**Clinical Pearl / High-Yield Fact**
When calculating Quetelet's index, it is essential to use the correct units for weight (kilograms) and height (meters) to ensure an accurate calculation. Additionally, the square of the height is a critical component of the formula, as it accounts for the proportional relationship between body mass and height.

**Correct Answer: C. weight (in kg) / height (in meters)²**