**Core Concept**
Gene transformation is a process by which bacteria take up free DNA from their surroundings, which can lead to genetic variation and exchange of genetic material between bacteria. This concept is crucial in understanding bacterial genetics, evolution, and the transfer of antibiotic resistance genes.

**Why the Correct Answer is Right**
The discovery of gene transformation is attributed to Frederick Griffith, who observed that a non-pathogenic strain of Streptococcus pneumoniae could transform into a pathogenic strain by acquiring a capsular polysaccharide gene from a virulent strain. This process occurs through a natural competence mechanism, where the bacteria take up free DNA from their environment. The acquired DNA is then integrated into the bacterial genome, resulting in the expression of new traits.

**Why Each Wrong Option is Incorrect**
* **Option A:** Escherichia coli (E. coli) is a well-studied bacterium, but it is not the primary organism associated with the discovery of gene transformation.
* **Option B:** Bacillus subtilis is a soil-dwelling bacterium that has been extensively studied for its genetics and physiology, but it is not the key organism in the discovery of gene transformation.
* **Option C:** Pseudomonas aeruginosa is an opportunistic pathogen, but it is not the bacterium linked to the discovery of gene transformation.

**Clinical Pearl / High-Yield Fact**
Gene transformation is an essential mechanism in bacterial genetics, allowing for the exchange of genetic material and contributing to the development of antibiotic resistance. Understanding this process is crucial in the field of microbiology and molecular biology.

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