**Core Concept**
The question is testing knowledge of the composition of dental plaque, a biofilm formed on teeth surfaces. Dental plaque contains various microorganisms, including bacteria, viruses, and fungi, which contribute to tooth decay and periodontal disease. The specific species found in adult plaque is of interest.

**Why the Correct Answer is Right**
The correct answer is **Sp.** (Species) **Porphyromonas gingivalis**. This anaerobic, Gram-negative rod is a key periodontal pathogen found in abundance in adult plaque. It produces virulence factors such as gingipains, which contribute to the breakdown of the periodontal tissue. Porphyromonas gingivalis is associated with chronic periodontitis and has been linked to other systemic diseases, including cardiovascular disease and diabetes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus mutans is a cariogenic bacterium, but it is more commonly associated with the formation of dental plaque in children rather than adults.
* **Option B:** Escherichia coli is a Gram-negative rod that can be found in the oral cavity, but it is not typically associated with periodontal disease in adults.
* **Option D:** Candida albicans is a fungus that can be found in the oral cavity, but it is more commonly associated with oral thrush rather than periodontal disease.

**Clinical Pearl / High-Yield Fact**
Porphyromonas gingivalis is a significant periodontal pathogen that can contribute to chronic periodontitis and other systemic diseases. Its presence in adult plaque highlights the importance of maintaining good oral hygiene and regular dental check-ups to prevent periodontal disease.

**Correct Answer:** C. Porphyromonas gingivalis