**Core Concept**
The question is related to **dental anatomy** and **orthodontics**, specifically focusing on **tooth impaction**. Tooth impaction occurs when a tooth cannot erupt into the dental arch due to various reasons such as overcrowding, abnormal tooth size, or obstruction by other teeth.

**Why the Correct Answer is Right**
The most frequent tooth to be impacted is the **third molar**, also known as the wisdom tooth. This is due to the limited space available in the jaw for the third molar to erupt, especially in a crowded dental arch. The **mandibular third molar** is more commonly impacted than the maxillary third molar.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the first molar is essential for the occlusion and is rarely impacted.
**Option B:** The second molar can be impacted, but it is less common than the third molar.
**Option C:** The canine is crucial for esthetics and function, and its impaction is less frequent than that of the third molar.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor the development of third molars, as impacted teeth can lead to **pericoronitis**, **cyst formation**, or **resorption of adjacent teeth**.

**Correct Answer:** D. The third molar.