**Core Concept**
The periodontal ligament (PDL) is a crucial structure surrounding the tooth, playing a vital role in maintaining the tooth's attachment to the alveolar bone. In cases of dental trauma, such as avulsion, preserving PDL viability is essential for successful reimplantation.

**Why the Correct Answer is Right**
To maintain PDL viability, the tooth should be stored in an environment that closely mimics the physiological conditions of the oral cavity. Milk is an excellent medium for this purpose. It provides a suitable pH, temperature, and nutrient-rich environment for the PDL cells. The lactose in milk also helps to prevent bacterial growth and maintains the PDL cells in a viable state.

**Why Each Wrong Option is Incorrect**
**Option A:** Water is not an ideal storage medium for avulsed teeth. It lacks the necessary nutrients and can cause the PDL cells to become desiccated, leading to cell death.
**Option B:** Saline solution is not a suitable medium for storing avulsed teeth. While it may help to maintain some PDL cell viability, it lacks the nutrients and buffering capacity of milk.
**Option C:** Tap water is not recommended for storing avulsed teeth. It may contain bacteria, heavy metals, and other contaminants that can damage the PDL cells.

**Clinical Pearl / High-Yield Fact**
In cases of dental trauma, time is of the essence. The tooth should be gently rinsed with cold milk or a commercial tooth preservation medium, and then stored in the same medium at room temperature. The sooner the tooth is reimplanted, the better the chances of successful reattachment and preservation of PDL viability.

**Correct Answer:** D. Milk.