**Core Concept**
The periodontal ligament (PDL) is a complex fibrous structure that connects the tooth to the surrounding alveolar bone, playing a crucial role in tooth support and sensation. The PDL consists of various types of fibers, each with distinct characteristics and functions.

**Why the Correct Answer is Right**
The principal fibers of the periodontal ligament are the **horizontal fibers**. These fibers run parallel to the long axis of the tooth, providing resistance to occlusal forces and helping to maintain the tooth's position within the jaw. The horizontal fibers are composed of type I collagen, which is a strong and flexible protein that allows for the transmission of forces between the tooth and the alveolar bone.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sharpey's fibers are the remnants of the periodontal ligament fibers that are embedded in the cementum of the tooth, but they are not the principal fibers of the PDL.
* **Option B:** Oblique fibers are a type of periodontal ligament fiber that runs at an angle to the long axis of the tooth, providing additional support and resistance to forces. However, they are not the principal fibers.
* **Option D:** Alveolar bone is the dense, spongy tissue that surrounds the tooth and provides attachment sites for the periodontal ligament fibers, but it is not the principal fibers themselves.

**Clinical Pearl / High-Yield Fact**
The periodontal ligament is a dynamic structure that is constantly adapting to changes in the mouth, such as tooth movement or orthodontic treatment. Understanding the anatomy and function of the PDL is essential for diagnosing and treating periodontal diseases.

**Correct Answer:** C