## Core Concept
The question pertains to the identification of bacteria based on their ability to produce pigments. Certain bacteria are known to produce pigments that can be detected on agar plates, which is a useful characteristic for their identification.

## Why the Correct Answer is Right
The correct answer, *C. Pseudomonas aeruginosa*, is known for producing pigments such as **pyocyanin** (blue-green pigment), **pyoverdin** (fluorescent pigment), and **pyorubin** (red-brown pigment). These pigments are responsible for the characteristic color of the colonies. *Pseudomonas aeruginosa* is a Gram-negative bacterium that can cause various infections, particularly in immunocompromised individuals.

## Why Each Wrong Option is Incorrect
* **Option A:** *Staphylococcus aureus* does not typically produce pigment; it is identified by other characteristics such as **coagulase positivity** and **golden-yellow colonies** on certain media, but it is not known for pigment production in the same way *Pseudomonas* species are.
* **Option B:** *Escherichia coli* does not produce pigment; it is often identified by its **rapid lactose fermentation** and does not have the characteristic pigmentation associated with *Pseudomonas* species.
* **Option D:** *Bacillus subtilis* does not produce pigment; it is known for its **ability to form spores** and is often used in laboratory experiments to demonstrate spore formation.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that *Pseudomonas aeruginosa* is often associated with **blue-green pus** due to the production of **pyocyanin**. This pigment can be seen in clinical specimens and cultures, making it a distinctive feature for identification.

## Correct Answer Line
**Correct Answer: C. Pseudomonas aeruginosa.**