**Core Concept**
Secondary cementum is a type of cementum that forms after the completion of root development and is typically found in areas where the tooth is subjected to heavy occlusal forces. It is characterized by a more compact and thicker structure compared to primary cementum.

**Why the Correct Answer is Right**
The formation of secondary cementum is a response to the mechanical stresses exerted on the tooth, particularly in areas such as the apical third of the root. This type of cementum helps to anchor the tooth to the surrounding bone and provides additional support against occlusal forces. The deposition of secondary cementum is mediated by the activity of cementoblasts, which are specialized cells that lay down the cementum matrix.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because secondary cementum is not typically found in the coronal third of the root, which is subjected to lighter occlusal forces.
* **Option B:** This option is incorrect because secondary cementum is not confined to the cervical third of the root, which is actually characterized by a thin layer of primary cementum.
* **Option D:** This option is incorrect because while secondary cementum can be found in areas of high occlusal stress, it is not exclusively confined to the apical third of the root.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the formation of secondary cementum is a dynamic process that can be influenced by various factors, including tooth position, occlusal forces, and periodontal health. Understanding the characteristics and distribution of secondary cementum is crucial for diagnosing and managing periodontal diseases.

**Correct Answer: C. The apical third of the root.**