**Core Concept**
The gingival index is a widely used method for assessing the severity of gingivitis, a common periodontal disease. It involves scoring the gingiva based on its appearance and response to probing. The index helps in evaluating the effectiveness of oral hygiene practices and the progression of the disease.

**Why the Correct Answer is Right**
The gingival index score 2 is characterized by bleeding on slight provocation. This indicates that the gingiva is inflamed and tender to the touch, but not severely so. The inflammation is primarily due to the accumulation of plaque, a biofilm of bacteria, on the teeth. The gingival epithelium, a thin layer of cells covering the gingiva, becomes edematous and more susceptible to bleeding. The presence of neutrophils, a type of white blood cell, in the gingival sulcus (the small space between the tooth and the gingiva) further contributes to the inflammation.

**Why Each Wrong Option is Incorrect**
**Option B:** Spontaneous bleeding is typically associated with a gingival index score of 3, where the gingiva is severely inflamed and bleeds easily without any provocation.
**Option C:** Colour change in gingiva is a feature of a gingival index score of 1, where the gingiva appears red, swollen, and bleeds on probing.
**Option D:** No change in gingiva is a feature of a healthy gingiva, which corresponds to a gingival index score of 0.

**Clinical Pearl / High-Yield Fact**
A gingival index score of 2 is often used as a threshold for initiating dental treatment, such as scaling and root planing, to prevent the progression of gingivitis to periodontitis, a more severe periodontal disease.

**✓ Correct Answer: A. Bleeding on slight provocation**