**Core Concept**
The primary teeth, also known as deciduous teeth, typically start erupting when a child is around 6 months old and continue to erupt until they are about 3 years old. As the child grows, the primary teeth gradually fall out and are replaced by permanent teeth, a process known as shedding. However, in some cases, one or more primary teeth may not fall out and remain in the jaw, a condition known as retention.

**Why the Correct Answer is Right**
The primary tooth that is most commonly retained is the second molar. This is because the second molar has a smaller root canal and a more complex root structure, which makes it more difficult for the tooth to fall out naturally. Additionally, the second molar often has a more direct path for the permanent tooth to erupt, which can cause the primary tooth to become impacted and retained.

**Why Each Wrong Option is Incorrect**
**Option A:** The first molar is not the most commonly retained primary tooth, as it usually falls out around the age of 10-12 years. The first molar has a larger root canal and a simpler root structure compared to the second molar.
**Option B:** The central incisor is not typically retained, as it usually falls out around the age of 6-7 years. The central incisor has a relatively small root and a straightforward path for the permanent tooth to erupt.
**Option C:** The canine is not commonly retained, as it usually falls out around the age of 9-10 years. The canine has a relatively long and pointed root, which makes it less likely to be retained.

**Clinical Pearl / High-Yield Fact**
The retention of a primary tooth can lead to crowding of the permanent teeth, as the permanent tooth may not have enough space to erupt properly. This can lead to orthodontic issues and may require intervention from an orthodontist.

**Correct Answer: C. Canine is not commonly retained.**