**Core Concept:**
Head gear is a dental device used to apply controlled forces to teeth during orthodontic treatment. It is designed to exert mechanical stress on teeth, which leads to remodeling of the alveolar bone and movement of teeth to achieve proper dental alignment and occlusion.

**Why the Correct Answer is Right:**
The correct answer is **Option C: Reduction in overjet**.

Head gear works by exerting forces on the teeth that are perpendicular to the long axis of the teeth, causing the teeth to move in a controlled manner. In the case of reducing overjet (the distance between the incisors of the upper arch and the lower lip), head gear is positioned such that the force is applied at the level of the anterior teeth, causing them to move posteriorly and downward, which reduces the overjet.

**Why Each Wrong Option is Incorrect:**

**Option A: Increase in overbite** is incorrect because the force applied by the head gear is directed at the incisors, causing them to move posteriorly and downward, which results in a reduction of overjet and increase in overbite.

**Option B: Increase in overjet** is incorrect because the force applied by the head gear is directed at the incisors, causing them to move posteriorly and downward, which results in a reduction of overjet.

**Option D: No effect on overjet** is incorrect because the force applied by the head gear is directed at the incisors, causing them to move posteriorly and downward, which results in a reduction of overjet.

**Clinical Pearl:**
The correct use of head gear in orthodontic treatment aims to achieve proper dental alignment and occlusion. Understanding the direction and application of forces by the head gear is crucial for successful treatment outcomes.