**Core Concept**
Dental resorption is a pathological process involving the destruction of dental hard tissues, primarily affecting the cementum covering the roots of teeth. It can be caused by various factors, including mechanical, chemical, or biological agents. This process is a significant concern in periodontal diseases and dental trauma.

**Why the Correct Answer is Right**
The correct answer is related to the action of **cystic fluid**, which is rich in enzymes such as **cathepsin B** and **matrix metalloproteinases (MMPs)**. These enzymes break down the organic and inorganic components of dental tissue, leading to its resorption. The cystic fluid is produced by the odontoclasts, which are multinucleated cells responsible for the resorption of dental hard tissues. The resorption process is a complex, multi-step process involving the activation of various enzymes and the degradation of the extracellular matrix.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the resorption of dental tissue is not primarily caused by the action of **osteoclasts**, which are responsible for bone resorption.

**Option B:** This option is incorrect because **osteoblasts** are involved in the formation of bone and dental tissue, not their resorption.

**Option C:** This option is incorrect because **periodontal ligament (PDL) cells** play a role in the maintenance and repair of the PDL, but they are not primarily responsible for the resorption of dental tissue.

**Clinical Pearl / High-Yield Fact**
The main difference between osteoclasts and odontoclasts is their location and function: osteoclasts resorb bone tissue, whereas odontoclasts resorb dental hard tissue.

**Correct Answer: D. Cystic fluid.