**Core Concept**
Gustafson's method is a technique used to estimate the age of human teeth by analyzing the root resorption, enamel wear, and other morphological changes. This method relies on the observation of various physical changes that occur in teeth over time, allowing for an estimate of the individual's age at the time of death.

**Why the Correct Answer is Right**
The most reliable criteria in Gustafson's method is the **cementum apposition**. Cementum is a thin layer of bone-like tissue that covers the root of the tooth, and its apposition (or growth) is a reliable indicator of age. As we age, cementum continues to form in layers, allowing for an accurate estimate of age based on the number of cementum layers present. This process is less affected by environmental factors and is a more consistent indicator of age compared to other criteria such as root resorption or enamel wear.

**Why Each Wrong Option is Incorrect**
**Option A:** Root resorption is a significant indicator of age in Gustafson's method, but it is not the most reliable criteria due to its variability among individuals. Root resorption can be influenced by factors such as genetics, diet, and oral hygiene.
**Option B:** Enamel wear is another criterion used in Gustafson's method, but it is less reliable than cementum apposition due to its susceptibility to environmental factors and individual variability.
**Option C:** Periodontal pocket depth is not a reliable indicator of age in Gustafson's method, as it is influenced by factors such as oral hygiene, diet, and gum disease.

**Clinical Pearl / High-Yield Fact**
Gustafson's method is a useful tool for forensic odontology, allowing for the estimation of age at death in individuals where other methods of age determination are not possible. However, it is essential to consider the limitations and variability of this method, especially when making estimates in forensic investigations.

**Correct Answer: C. Cementum apposition. Cementum apposition is the most reliable criteria in Gustafson's method of identification.**