## Core Concept
The question tests the knowledge of bacterial cell wall structure and its relation to Gram staining. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain used in Gram staining, making them appear purple under a microscope. Bacillus species are known to be Gram-positive.

## Why the Correct Answer is Right
The correct answer, , is *Bacillus subtilis*, a species of Gram-positive, rod-shaped bacteria. It is commonly found in soil and is known for its ability to form highly resistant, dormant spores. The cell wall of *Bacillus subtilis* contains a thick layer of peptidoglycan, which is responsible for its Gram-positive staining.

## Why Each Wrong Option is Incorrect
* **Option A:** *Escherichia coli* is a Gram-negative bacterium. It has a thinner peptidoglycan layer and an outer membrane containing lipopolysaccharides, which does not retain the crystal violet stain.
* **Option B:** *Pseudomonas aeruginosa* is also a Gram-negative bacterium. It has a similar cell wall structure to *E. coli*, with a thinner peptidoglycan layer and an outer membrane.
* **Option D:** *Streptococcus pneumoniae* is a Gram-positive coccus, not a bacillus. While it does retain the crystal violet stain, its cellular morphology is different from that of bacilli.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Bacillus anthracis*, another species of Bacillus, is a significant pathogen that causes anthrax. It is also Gram-positive and rod-shaped but has distinct colonial and microscopic features.

## Correct Answer: C. Bacillus subtilis