**Core Concept**
Plasmids are small, self-replicating circular DNA molecules that play a crucial role in bacterial conjugation. Conjugation is a process of horizontal gene transfer between bacteria, where a donor bacterium transfers its plasmid to a recipient bacterium through direct cell-to-cell contact or a bridge-like structure.

**Why the Correct Answer is Right**
Lederberg and Tatum's pioneering work on conjugation involved the study of genetic recombination in bacteria. They discovered that the ability of bacteria to transfer genetic material was linked to the presence of plasmids. In their experiments, they used Escherichia coli (E. coli) as the model organism to demonstrate the role of plasmids in conjugation. The plasmid-mediated transfer of genetic information in E. coli led to the development of the "sex-factor" hypothesis, which posits that plasmids are essential for conjugation.

**Why Each Wrong Option is Incorrect**
**Option A:** H.influenzae (Haemophilus influenzae) is a Gram-negative bacterium that causes infections such as meningitis and otitis media. While conjugation occurs in H. influenzae, it was not the primary organism used by Lederberg and Tatum to describe the role of plasmids in conjugation.
**Option B:** Corynebacterium is a genus of Gram-positive bacteria that includes species such as Corynebacterium diphtheriae, the causative agent of diphtheria. Although conjugation has been observed in some Corynebacterium species, it was not the focus of Lederberg and Tatum's work.
**Option C:** Pseudomonas is a genus of Gram-negative bacteria that includes species such as Pseudomonas aeruginosa, a common pathogen in cystic fibrosis patients. While conjugation occurs in Pseudomonas species, it was not the primary organism used by Lederberg and Tatum to describe the role of plasmids in conjugation.

**Clinical Pearl / High-Yield Fact**
The discovery of conjugation by Lederberg and Tatum revolutionized our understanding of bacterial genetics and horizontal gene transfer. It led to the development of new approaches for the study of bacterial genetics and the identification of virulence factors in pathogenic bacteria.

**✓ Correct Answer: D. Esch.Coli Escherichia coli**