**Core Concept**
The alveolar crest is the most apical point of the alveolar bone surrounding the roots of teeth. Its level in relation to the cemento-enamel junction (CEJ) of adjacent teeth is a critical parameter in assessing periodontal health and diagnosing periodontal defects.

**Why the Correct Answer is Right**
The alveolar crest should be at the same level as the CEJ of adjacent teeth in a healthy periodontium. This is because the alveolar bone and the CEJ of adjacent teeth are part of the same anatomical unit, and any discrepancy may indicate periodontal disease or other pathological conditions. The correct level of the alveolar crest is essential in evaluating the periodontal status and planning appropriate treatment.

**Why Each Wrong Option is Incorrect**
* **Option B:** This option is incorrect because a level of alveolar crest above the CEJ of adjacent teeth may indicate periodontal disease, specifically a vertical bone loss.
* **Option C:** This option is incorrect because a level of alveolar crest below the CEJ of adjacent teeth may indicate a periodontal defect or a horizontal bone loss.

**Clinical Pearl / High-Yield Fact**
It is essential to assess the level of the alveolar crest in relation to the CEJ of adjacent teeth during periodontal examination, as it provides valuable information about the periodontal health and aids in the diagnosis of periodontal defects.

**Correct Answer: D. Level of alveolar crest below the CEJ of the adjacent teeth.**