**Core Concept**
Gingivitis is an inflammatory disease of the gingiva, characterized by the accumulation of microbial plaque and the subsequent activation of the host's immune response. The clinical manifestations of gingivitis are primarily due to the release of inflammatory mediators and the infiltration of immune cells into the gingival tissues.

**Why the Correct Answer is Right**
The most objective sign of gingivitis is the presence of bleeding on probing (BOP), which is a direct indicator of the inflammation and damage to the gingival tissues. This is due to the increased vascular permeability and the release of histamine, serotonin, and other inflammatory mediators that cause the blood vessels to dilate and become more permeable. The gingival crevicular fluid (GCF) also increases in volume, making the gingiva more susceptible to bleeding when subjected to minor trauma, such as probing.

**Why Each Wrong Option is Incorrect**
* **Option A:** Gingival recession is a common consequence of advanced periodontitis, but it is not an objective sign of gingivitis.
* **Option B:** Bad breath (halitosis) is a symptom of gingivitis, but it is subjective and can be influenced by various factors, making it an unreliable indicator.
* **Option D:** Swollen gingiva is a clinical sign of gingivitis, but it is not as objective as bleeding on probing, as it can be influenced by various factors, such as edema and inflammation.

**Clinical Pearl / High-Yield Fact**
The "Bleeding on Probing" index is a simple and reliable method for assessing the severity of gingivitis. It involves gently probing the gingival sulcus with a periodontal probe and observing for bleeding. A bleeding index of 0-20% indicates healthy gingiva, while an index of 21-40% indicates mild gingivitis, and an index of 41-60% indicates moderate gingivitis.

**Correct Answer: C. Bleeding on probing**