## **Core Concept**
The question tests knowledge of the common species of streptococci found in human dental plaque. Streptococci are a diverse group of bacteria, and their presence in dental plaque is significant for oral health and disease.

## **Why the Correct Answer is Right**
Streptococcus mutans (not listed but relevant for context) and other species like *Streptococcus sanguinis*, *Streptococcus oralis*, and *Streptococcus mitis* are commonly found in human dental plaque. *Streptococcus pneumoniae* is primarily known for causing pneumonia and is not typically considered a resident flora of dental plaque. Instead, it's more commonly associated with respiratory infections.

## **Why Each Wrong Option is Incorrect**
* **Option A:** *Streptococcus mutans* is indeed a common inhabitant of dental plaque and plays a significant role in tooth decay.
* **Option B:** *Streptococcus sanguinis* is also commonly found in dental plaque and is known for its role in the development of dental caries and endocarditis.
* **Option C:** This option might represent another common streptococcal species found in the oral cavity, potentially *Streptococcus oralis* or *Streptococcus mitis*, which are both normal inhabitants of the oral cavity.
* **Option D:** If this option represents *Streptococcus pneumoniae*, it is less commonly associated with dental plaque compared to the others listed.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while various streptococcal species inhabit the oral cavity, *Streptococcus pneumoniae* is more commonly linked with infections like pneumonia rather than being a typical resident of dental plaque.

## **Correct Answer:** D. *Streptococcus pneumoniae*.