**Core Concept:** R-factors are a type of plasmid in bacteria that can be horizontally (gene) transmitted between bacterial cells. Plasmids are small, circular, self-replicating DNA molecules that can exist independently of the cell's chromosomal DNA. R-factors carry antibiotic resistance genes, which confer resistance to specific antibiotics to the bacteria that acquire them.

**Why the Correct Answer is Right:** R-factors are transmissible elements that can move between bacterial cells, allowing the spread of antibiotic resistance genes. In this case, the correct answer is **D**. DNA is the genetic material that carries the antibiotic resistance genes on the R-factor. DNA can be taken up by bacterial cells through a process called conjugation, where the DNA is exchanged directly between two bacterial cells. In the context of R-factors, this means that the antibiotic resistance genes can be transmitted from one bacteria to another.

**Why Each Wrong Option is Incorrect:**

A. Option A (plasmids) is incorrect because although plasmids can carry antibiotic resistance genes, they are not the mode of transmission. Transmission occurs through conjugation, not plasmids.

B. Option B (transposons) are another type of genetic element in bacteria that can move between cells, but unlike R-factors, transposons do not carry antibiotic resistance genes.

C. Option C (viruses) is incorrect because viruses primarily infect and replicate within bacterial cells, causing lysis and releasing viral particles. They do not enable antibiotic resistance gene transmission between bacterial cells.

**Clinical Pearl / High-Yield Fact:** The correct answer, **D** (DNA), highlights the importance of understanding horizontal gene transmission in antibiotic resistance, which is a major clinical concern in the fight against bacterial infections. Understanding these mechanisms helps guide antibiotic usage and informs treatment strategies to combat antibiotic-resistant bacteria.