**Core Concept**
Tooth mobility is a clinical indicator of periodontal disease, and assessing it is crucial in the diagnosis and management of periodontal patients. The periodontal ligament (PDL) connects the tooth to the surrounding alveolar bone, and its integrity is essential for maintaining tooth stability.

**Why the Correct Answer is Right**
The best indicator of tooth mobility is the **Horizontal Mobility Index (HMI)**, which measures the degree of horizontal tooth movement. This is assessed by applying a gentle force to the tooth in a buccal-lingual direction, and the resulting movement is measured. The HMI is a more accurate indicator of tooth mobility than vertical movement, as it reflects the PDL's ability to resist horizontal forces.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **Vertical Mobility Index (VMI)** measures the degree of vertical tooth movement, which is not a reliable indicator of tooth mobility. While VMI can indicate the presence of a loose tooth, it does not accurately reflect the PDL's integrity.
* **Option B:** **Tooth sensitivity** is a common symptom of periodontal disease, but it is not a reliable indicator of tooth mobility. Sensitivity can be caused by various factors, including pulpitis, and does not necessarily correlate with the degree of tooth mobility.
* **Option C:** **Tooth color** can be an indicator of various oral health conditions, but it is not a reliable indicator of tooth mobility. Tooth color can be affected by various factors, including diet, hygiene, and systemic conditions.

**Clinical Pearl / High-Yield Fact**
When assessing tooth mobility, it's essential to remember that a single measurement may not accurately reflect the PDL's integrity. A more accurate assessment can be obtained by measuring tooth mobility in multiple directions and considering the patient's overall periodontal status.

**Correct Answer: C. Tooth color**