**Core Concept**
Impaction of a third molar (wisdom tooth) occurs when there is not enough space in the jaw for the tooth to erupt normally. This can lead to various complications, including pain, infection, and damage to surrounding teeth and tissues. The difficulty of impaction depends on several factors, including the angle of impaction, the presence of any surrounding bone or tissue, and the patient's overall oral health.

**Why the Correct Answer is Right**
The most difficult impaction of a third molar is typically considered to be a **Vertical Impaction**. This type of impaction occurs when the tooth is impacted in a downward direction, with the crown of the tooth pointing towards the floor of the mouth. This can make the tooth difficult to remove surgically, as it may require significant bone removal to access the tooth. Additionally, vertical impactions can be associated with a higher risk of nerve damage, particularly to the inferior alveolar nerve, which runs through the jawbone and supplies sensation to the lower teeth and chin.

**Why Each Wrong Option is Incorrect**
* **Option A: Horizontal Impaction** is incorrect because while it can be challenging, it is generally considered less difficult than a vertical impaction. Horizontal impactions occur when the tooth is impacted in a side-to-side direction, with the crown of the tooth pointing towards the cheek or tongue.
* **Option B: Mesial Impaction** is incorrect because it refers to an impaction where the tooth is impacted towards the front of the mouth, rather than downwards. This type of impaction is generally considered less difficult than a vertical impaction.
* **Option C: Distal Impaction** is incorrect because it refers to an impaction where the tooth is impacted towards the back of the mouth, rather than downwards. This type of impaction is generally considered less difficult than a vertical impaction.

**Clinical Pearl / High-Yield Fact**
It's essential for dentists and oral surgeons to carefully evaluate the position and angulation of impacted third molars before attempting removal, as the difficulty of the procedure can have significant implications for patient outcomes and recovery.

**Correct Answer: C. Distal Impaction**