**Core Concept**
Root completion in primary teeth is a critical aspect of pediatric dentistry and orthodontics. It refers to the process of root formation and maturation in primary teeth, which is essential for the overall development and health of the tooth.

**Why the Correct Answer is Right**
Root completion in primary teeth typically occurs between 8-12 years of age. This process involves the gradual formation of the root structure, which is influenced by the activity of odontoblasts, the cells responsible for dentin production. The rate of root completion is influenced by factors such as genetics, nutrition, and overall health.

**Why Each Wrong Option is Incorrect**
**Option A:** Root completion in primary teeth is not typically completed by the age of 5, as this is still within the range of primary tooth development.
**Option B:** Root completion in primary teeth is not typically completed by the age of 15, as this is closer to the age of permanent tooth eruption.
**Option C:** Root completion in primary teeth is not typically completed by the age of 6, as this is still within the range of primary tooth development.

**Clinical Pearl / High-Yield Fact**
It's essential to note that root completion in primary teeth is a crucial factor in determining the timing of primary tooth loss and the subsequent eruption of permanent teeth. A delay in root completion can lead to an increased risk of tooth decay and other oral health issues.

**Correct Answer: C. 8-12 years of age**