## Core Concept
Gustafson's method is used for age determination in adults based on the changes in teeth over time. This method evaluates several characteristics of teeth to estimate age.

## Why the Correct Answer is Right
The correct answer, **Attrition**, is considered the most important sign because tooth wear occurs consistently over time due to mechanical forces and erosion. Gustafson's criteria include attrition, cementum apposition, root resorption, transparency of the root, secondary dentin formation, cementum annular formation, and root morphology, but attrition is a direct and observable measure of aging.

## Why Each Wrong Option is Incorrect
- **Option A:** Although **Cementum apposition** is one of Gustafson's criteria, it is not considered the most important sign. Cementum apposition can be influenced by various factors, including environmental and pathological conditions.
- **Option B:** **Root resorption** is another criterion but is less reliable for age estimation due to its variability among individuals, often influenced by pathological processes rather than age alone.
- **Option D:** **Transparency of the root** is also a criterion, but its measurement can be challenging and less directly correlated with age compared to attrition.

## Clinical Pearl / High-Yield Fact
A key point to remember is that Gustafson's method provides an estimate of age and is often used in forensic odontology. While attrition is a critical factor, the accuracy of age estimation can be affected by diet, oral hygiene, and environmental factors.

## Correct Answer: C. Attrition.