**Core Concept**
During orthodontic treatment, various biochemical reactions occur in response to the applied forces, leading to tooth movement. These reactions involve the breakdown of the periodontal ligament (PDL) and the surrounding bone tissue.

**Why the Correct Answer is Right**
The correct answer involves the understanding of the biochemical reactions involved in orthodontic tooth movement. When orthodontic forces are applied, it leads to the activation of osteoclasts, which break down the bone tissue, and the activation of osteoblasts, which form new bone tissue. This process involves the release of various biochemical mediators, such as prostaglandins, which stimulate the breakdown of the PDL and the surrounding bone tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Bone resorption is a common reaction during orthodontic treatment, making it an incorrect answer.
**Option B:** Increased osteoclastic activity is also a common reaction during orthodontic treatment, making it an incorrect answer.
**Option C:** Bone formation is a desired outcome during orthodontic treatment, making it an incorrect answer.

**Clinical Pearl / High-Yield Fact**
The rate and extent of tooth movement can be influenced by various factors, including the magnitude and duration of the applied forces, the presence of periodontal disease, and the patient's overall health status.

**Correct Answer:** D. Bone formation.