**Core Concept**
Gingival probing is a diagnostic tool used to assess the severity of periodontal disease. In gingivitis, the inflammation of the gingiva is the primary concern, and measuring the pocket depth is crucial to determine the extent of the disease.

**Why the Correct Answer is Right**
When performing a periodontal probing in a patient with gingivitis, the depth is measured from the free gingival margin (FGM) to the bottom of the gingival sulcus. This measurement helps to identify the presence and severity of gingival inflammation. The free gingival margin is the edge of the gingiva that lies at the junction of the gingiva and the tooth. The periodontal probe is gently inserted into the gingival sulcus, and the depth is measured at multiple sites to assess the overall health of the periodontium.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the free gingival margin is the correct reference point for measuring the pocket depth in gingivitis.
* **Option B:** This option is incorrect because the alveolar crest is the bony reference point for measuring the pocket depth, but it is not the correct reference point for gingivitis.
* **Option C:** This option is incorrect because the cemento-enamel junction (CEJ) is the boundary between the enamel and cementum, and it is not the correct reference point for measuring the pocket depth in gingivitis.
* **Option D:** This option is incorrect because the gingival crest is the most apical part of the free gingiva, but it is not the correct reference point for measuring the pocket depth in gingivitis.

**Clinical Pearl / High-Yield Fact**
The free gingival margin is the most critical reference point for measuring the pocket depth in gingivitis, as it allows for accurate assessment of the extent of gingival inflammation.

**Correct Answer:** A.