## Core Concept
The question pertains to the historical experiment by Frederick Griffith that demonstrated the concept of bacterial transformation. This experiment involved the transfer of genetic material from one bacterium to another, resulting in a change in the recipient bacterium's properties. The core concept here is the **transformation of bacterial strains**, which laid the foundation for the understanding of genetic exchange in bacteria.

## Why the Correct Answer is Right
Frederick Griffith's experiment in 1928 used **Streptococcus pneumoniae** (pneumococcus) to demonstrate transformation. He showed that a non-virulent strain of *S. pneumoniae* could be transformed into a virulent strain by exposure to heat-killed virulent bacteria. The correct answer, *. pneumoniae*, is right because Griffith's experiment specifically involved this species to illustrate the phenomenon of bacterial transformation, where the non-virulent (rough) strain acquired a polysaccharide capsule and became virulent (smooth) after exposure to the heat-killed virulent strain.

## 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 in Griffith's experiment.
- **Option B:** This option is incorrect as there is no well-known bacterium directly associated with Griffith's experiment by this name, and it does not pertain to the historical context of Griffith's findings.
- **Option D:** This option is incorrect because *Bacillus subtilis* is another bacterium used in various genetic studies but was not the subject of Griffith's transformation experiment.

## Clinical Pearl / High-Yield Fact
A key point to remember is that Griffith's experiment was pivotal in establishing that DNA is the genetic material. Later work by Avery, MacLeod, and McCarty built upon Griffith's findings and identified DNA as the molecule responsible for bacterial transformation. This historical context is crucial for understanding the development of molecular biology.

## Correct Answer: B. .