**Core Concept**
Gingivitis is an inflammatory condition of the gingiva, characterized by the accumulation of plaque and the resultant host response, which can lead to the destruction of the gingival connective tissue and the junctional epithelium. Radiographically, gingivitis can manifest as a subtle change in the gingival contour or as an increase in the interdental bone density.

**Why the Correct Answer is Right**
The correct answer is related to the radiographic appearance of gingivitis, which is often characterized by an increase in the interdental bone density due to the accumulation of plaque and the resultant inflammation. This increase in bone density is a result of the deposition of new bone in response to the inflammatory stimulus. The increased bone density can be visualized on radiographs, making it a key diagnostic feature of gingivitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because gingivitis typically does not result in the loss of interdental bone density, but rather an increase due to the inflammatory response.
**Option B:** This option is incorrect because apical periodontitis is a separate entity that is characterized by inflammation of the periapical tissues, often in response to pulp necrosis.
**Option C:** This option is incorrect because periodontitis is a more advanced stage of periodontal disease that is characterized by the loss of interdental bone density and the destruction of the periodontal ligament.

**Clinical Pearl / High-Yield Fact**
It is essential to note that radiographic evidence of gingivitis may be subtle and may not always be present. Therefore, a thorough clinical examination, including the assessment of plaque accumulation and gingival inflammation, is crucial in the diagnosis of gingivitis.

**Correct Answer: D. Increase in interdental bone density.**