**Core Concept**
The primary dentition, also known as baby teeth, begins erupting when a child is around 6 months old and continues until they are about 3 years old. The primary dentition starts to exfoliate and is gradually replaced by the permanent dentition between the ages of 6 to 12 years.

**Why the Correct Answer is Right**
The average duration of function for primary dentition is approximately 6 years. This is because the primary teeth start to decay and become loose as the permanent teeth erupt, causing them to fall out. The primary teeth are smaller and less durable than permanent teeth, making them more susceptible to decay and wear. The eruption of permanent teeth typically starts in the front of the mouth and progresses to the back, with the first permanent teeth usually appearing around the age of 6 years.

**Why Each Wrong Option is Incorrect**
**Option B:** 8 years is incorrect because the primary dentition starts to exfoliate and is gradually replaced by the permanent dentition between the ages of 6 to 12 years, and the average duration of function is around 6 years.

**Option C:** 10 years is incorrect because the primary dentition starts to decay and become loose as the permanent teeth erupt, causing them to fall out, and the average duration of function is around 6 years.

**Option D:** 12 years is incorrect because the primary dentition is gradually replaced by the permanent dentition between the ages of 6 to 12 years, and the average duration of function is around 6 years.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the duration of function for primary dentition can vary depending on factors such as oral hygiene, diet, and overall health. Regular dental check-ups and proper oral care can help maintain the health of primary teeth and promote a smooth transition to the permanent dentition.

**✓ Correct Answer: A. 6 years**