**Core Concept**
The primary teeth, also known as deciduous teeth, play a crucial role in the development and maintenance of oral health in children. Understanding the duration of function of each primary tooth is essential for pediatric dentistry and orthodontics.

**Why the Correct Answer is Right**
The primary tooth with the longest duration of function is the first molar. This is because the first molars, specifically the lower first molars, often remain in the oral cavity until the age of 12-13 years. The prolonged presence of these teeth is due to their larger size and more complex root structure compared to other primary teeth. As a result, they provide a prolonged functional period, allowing for proper occlusal development and adequate space maintenance for the permanent teeth.

**Why Each Wrong Option is Incorrect**
* **Option A:** The central incisors, being the most anterior teeth, have a relatively short duration of function, typically lasting around 6-7 years.
* **Option B:** The lateral incisors, although larger than central incisors, also have a relatively short duration of function, typically lasting around 7-8 years.
* **Option C:** The canines, also known as cuspids, have a moderate duration of function, typically lasting around 8-9 years.

**Clinical Pearl / High-Yield Fact**
Remember that the duration of function of primary teeth can vary depending on factors such as oral hygiene, diet, and genetics. However, the first molars generally have the longest functional period among the primary teeth.

**Correct Answer:** C. Canines.