**Core Concept**
The 'Red complex' in periodontal disease is characterized by the presence of specific bacterial species that contribute to inflammation and tissue destruction. This complex is typically associated with the clinical manifestation of bleeding on probing.

**Why the Correct Answer is Right**
The Red complex is primarily composed of Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola. These bacteria are anaerobic and have a synergistic relationship, leading to the degradation of periodontal tissues and the characteristic bleeding on probing. P. gingivalis is particularly implicated in the pathogenesis of periodontitis, as it produces virulence factors such as gingipains, which facilitate bacterial invasion and tissue destruction.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the 'Orange complex' is a different group of bacteria associated with periodontal disease, primarily consisting of Aggregatibacter actinomycetemcomitans.

**Option B:**
This option is incorrect because the 'Yellow complex' is another group of bacteria, primarily consisting of Fusobacterium nucleatum.

**Option C:**
This option is incorrect because Actinobacillus actinomycetemcomitans is a bacterium associated with localized aggressive periodontitis, not the Red complex.

**Clinical Pearl / High-Yield Fact**
The Red complex is a key target for periodontal therapy, as eliminating these bacteria can significantly reduce inflammation and prevent disease progression.

**Correct Answer:** C. Treponema denticola