## **Core Concept**
The question pertains to the historical experiment demonstrating bacterial transformation, a pivotal concept in molecular biology. **Bacterial transformation** refers to the process by which bacteria take up free DNA molecules from their surroundings, leading to a change in genotype and phenotype. This concept is fundamental to understanding genetic exchange in bacteria.

## **Why the Correct Answer is Right**
Frederick Griffith's experiment in 1928 was crucial in demonstrating bacterial transformation. He worked with *Streptococcus pneumoniae*, which causes pneumonia. Griffith used two strains of the bacteria: a virulent (disease-causing) strain with a polysaccharide capsule (S strain) and a non-virulent strain without the capsule (R strain). The S strain has a smooth colony appearance due to its polysaccharide capsule, while the R strain has a rough appearance. Griffith showed that when he injected mice with a mixture of heat-killed S strain and live R strain, some mice died, and live S strain bacteria were recovered from them. This indicated that the R strain had been transformed into the S strain by acquiring the genetic material from the heat-killed S strain. The correct answer, *,* is associated with *Streptococcus pneumoniae*.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although *E. coli* is a commonly used organism in genetic studies, it was not used by Griffith to demonstrate transformation.
- **Option B:** This option is incorrect as there is no well-known association of transformation experiments with this organism in the context provided.
- **Option D:** This option is incorrect because, while *Bacillus subtilis* can undergo transformation, it was not the organism used by Griffith.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Griffith's experiment laid the groundwork for the discovery of DNA as the genetic material. **The Avery-MacLeod-McCarty experiment** later identified DNA as the substance responsible for bacterial transformation. This historical context is crucial for understanding the development of molecular biology.

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