**Core Concept**
The 'String of pearl' colonies on nutrient agar is a characteristic feature of certain bacteria, indicating their ability to grow in a specific pattern. This pattern is due to the production of a capsule or slime layer, which helps in the formation of these distinct colonies. The underlying principle being tested is the identification of bacteria based on their colonial morphology.

**Why the Correct Answer is Right**
The correct answer is related to the bacterial species that produces this unique colonial morphology. The "String of pearl" appearance is typically seen in *Streptococcus* species, particularly *Streptococcus pneumoniae*, due to their capsular polysaccharides. The capsule helps in the formation of these distinct colonies, which resemble a string of pearls.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correspond to the bacterial species known for producing "String of pearl" colonies.
**Option B:** This option is also incorrect as it does not match the characteristic colonial morphology described.
**Option C:** Similarly, this option is incorrect as it is not associated with the "String of pearl" appearance.
**Option D:** This option is incorrect because it is not the correct bacterial species that produces this unique colonial morphology.

**Clinical Pearl / High-Yield Fact**
The "String of pearl" colonies are a key diagnostic feature for identifying *Streptococcus pneumoniae*, which is a common cause of pneumonia and other invasive infections. Recognizing this colonial morphology is essential for laboratory diagnosis and guiding appropriate antibiotic therapy.

**Correct Answer:** D. Streptococcus pneumoniae