**Core Concept**
The center of rotation is a critical concept in orthodontics, referring to the point around which a tooth or a segment of the dental arch rotates during orthodontic movement. In the context of maxillary central incisor movement, the center of rotation is used to determine the direction and extent of tooth movement.

**Why the Correct Answer is Right**
When the center of rotation is applied to the apex (tip) of the maxillary central incisor, the tooth will exhibit **labial tipping**. This type of tooth movement involves a rotation around the apex, resulting in the labial (front) surface of the tooth moving in a labial direction. This movement is characterized by a minimal change in the vertical position of the tooth, with the majority of the movement occurring in the labial direction. The labial tipping is a common type of tooth movement used in orthodontic treatment to correct Class II or Class III malocclusions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the center of rotation applied to the apex of the maxillary central incisor.
**Option B:** This option may be related to a different type of tooth movement, but it is not the correct answer for the center of rotation applied to the apex.
**Option D:** This option is also not relevant to the center of rotation applied to the apex of the maxillary central incisor.

**Clinical Pearl / High-Yield Fact**
When treating a Class II malocclusion with a maxillary central incisor that is labially positioned, applying a center of rotation to the apex of the tooth can help to correct the labial inclination and improve the overall alignment of the dental arch.

**Correct Answer: C. labial tipping**