**Core Concept**
The gingival fibers are a group of collagen fibers that play a crucial role in maintaining the integrity of the periodontal tissues. They are classified into three main groups: the alveolar, the circular, and the transseptal fibers. The transseptal fibers are further divided into two subgroups: the transseptal fibers that insert into the cementum of adjacent teeth and the transgingival fibers that insert into the gingiva.

**Why the Correct Answer is Right**
The transseptal fibers that insert into the cementum of adjacent teeth are the ones that connect two adjacent teeth. This group of fibers is essential for maintaining the periodontal ligament and allowing for the proper transmission of forces during mastication. The insertion of these fibers into the cementum of adjacent teeth provides a strong bond between the teeth, preventing excessive mobility and ensuring proper occlusion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the alveolar fibers are the ones that insert into the alveolar bone, not into the cementum of adjacent teeth.
**Option B:** This option is incorrect because the circular fibers are the ones that encircle the tooth, providing a compressive force against the tooth.
**Option C:** This option is incorrect because the periodontal ligament (PDL) fibers are the ones that connect the tooth to the surrounding alveolar bone, not to adjacent teeth.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the gingival fibers play a crucial role in maintaining the integrity of the periodontal tissues and that the transseptal fibers are the ones that connect two adjacent teeth.

**Correct Answer:** D. Transseptal fibers that insert into the cementum of adjacent teeth.