**Core Concept**
The period of mixed dentition refers to the stage in dental development where both primary and permanent teeth are present in the oral cavity. This period is characterized by the simultaneous eruption of primary and permanent teeth, leading to a unique dental arch configuration.

**Why the Correct Answer is Right**
During the period of mixed dentition, primary teeth begin to exfoliate and are replaced by permanent teeth. This process typically starts around the age of 6 years and continues until the age of 12-14 years. The mixed dentition period is crucial for the development of proper occlusion and alignment of the teeth. **The permanent incisors usually start erupting during this period, and their proper alignment is essential for maintaining a healthy occlusal relationship.**

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct age range for the mixed dentition period.
**Option B:** This option is incorrect because it refers to the primary dentition period, which occurs before the eruption of permanent teeth.
**Option C:** This option is incorrect because it refers to the permanent dentition period, which occurs after the exfoliation of all primary teeth.

**Clinical Pearl / High-Yield Fact**
It is essential for dentists and orthodontists to assess the occlusal relationship during the mixed dentition period to prevent potential orthodontic problems in the future.

**Correct Answer: C. 6-12 years**