**Core Concept**
The question is testing the understanding of an age-independent index, which is a measure used in clinical practice to assess various physiological parameters without being influenced by age. This concept is crucial in evaluating the health and well-being of patients across different age groups.

**Why the Correct Answer is Right**
The corrected answer is **C. Body Mass Index (BMI)**. BMI is an age-independent index that calculates an individual's body fat based on their weight and height. It is calculated as weight (in kg) divided by the square of height (in meters). BMI takes into account the weight and height of an individual without being influenced by their age, making it a reliable measure for assessing body fat across different age groups.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Waist Circumference** is an age-dependent index, as waist circumference increases with age due to the accumulation of body fat.
* **Option B:** **Albumin-to-Creatinine Ratio** is a measure used to assess kidney function, but it can be influenced by age, as kidney function declines with age.
* **Option D:** **Triglyceride-to-HDL Ratio** is a measure used to assess cardiovascular risk, but it can also be influenced by age, as triglyceride levels and HDL levels change with age.

**Clinical Pearl / High-Yield Fact**
Remember that BMI is a useful age-independent index for assessing body fat, but it has its limitations. For example, BMI may not accurately reflect body fat in individuals with muscular builds or those who have a high percentage of body water.

**Correct Answer:** C. Body Mass Index (BMI)