**Core Concept**
The center of resistance (CR) is a critical concept in orthodontics, referring to the point where the forces applied to a tooth are transmitted to the surrounding bone and periodontal ligament. In the context of maxillary anterior teeth, the CR is an essential consideration for orthodontic treatment planning.

**Why the Correct Answer is Right**
The center of resistance of six maxillary anterior teeth is located at the gum line, between the central incisors. This is because the CR is influenced by the distribution of forces across the teeth, as well as the anatomy of the surrounding bone and periodontal ligament. In the maxillary anterior region, the CR tends to be more posterior and apical due to the presence of the alveolar bone and the direction of the forces applied to the teeth.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the CR is not located at the incisal edge, as the forces applied to the teeth are not transmitted to the surrounding bone and periodontal ligament at this point.
* **Option B:** This option is incorrect because the CR is not located at the midline of the maxillary arch, as the distribution of forces across the teeth is not symmetrical in this region.
* **Option C:** This option is incorrect because the CR is not located at the apical third of the teeth, as the forces applied to the teeth are not transmitted to the surrounding bone and periodontal ligament at this point.

**Clinical Pearl / High-Yield Fact**
The center of resistance is an essential concept in orthodontic treatment planning, and its location can significantly impact the outcome of treatment. Remember that the CR is influenced by the distribution of forces across the teeth, as well as the anatomy of the surrounding bone and periodontal ligament.

**Correct Answer:** C.