**Core Concept**
The ideal medium for transporting an avulsed tooth is a solution that maintains the vitality of the periodontal ligament cells and the tooth pulp. This requires a solution that is physiologically compatible with the tooth and its surrounding tissues.

**Why the Correct Answer is Right**
The correct answer is Hank's balanced salt solution (HBSS). HBSS is a physiologically balanced solution that maintains the ionic environment of the tooth and its surrounding tissues. It contains the essential ions, such as calcium, magnesium, and phosphate, which are necessary for the survival of periodontal ligament cells and the tooth pulp. When an avulsed tooth is placed in HBSS, it helps to preserve the vitality of the periodontal ligament cells, which is essential for successful replantation and reattachment.

**Why Each Wrong Option is Incorrect**
**Option A:** Water is not an ideal medium for transporting an avulsed tooth because it lacks essential ions and can cause the tooth to become edematous, leading to cell death.
**Option B:** Milk is not a suitable medium for transporting an avulsed tooth because it can cause the tooth to become calcified, leading to cell death. Additionally, milk is not a physiologically balanced solution.
**Option C:** Saline solution is not ideal for transporting an avulsed tooth because it lacks the essential ions and buffering capacity of HBSS, which can lead to cell death.

**Clinical Pearl / High-Yield Fact**
When transporting an avulsed tooth, it is essential to handle the tooth gently and place it in a container filled with HBSS to maintain the vitality of the periodontal ligament cells and the tooth pulp.

**Correct Answer:** D. Hank's balanced salt solution (HBSS)