**Core Concept**
R-factor (resistance factor) is a bacterial genetic element that confers antibiotic resistance. Its transfer between bacteria occurs via direct cell-to-cell contact, a process that relies on plasmid-mediated gene transfer.

**Why the Correct Answer is Right**
R-factor is a plasmid that carries genes for antibiotic resistance and is transferred through **conjugation**, a process involving the formation of a pilus between two bacterial cells. The donor cell transfers a copy of the R-factor to the recipient via a conjugation bridge, enabling horizontal gene transfer. This mechanism is distinct from transduction (bacteriophage-mediated) or transformation (free DNA uptake).

**Why Each Wrong Option is Incorrect**
Option A: Transduction involves bacteriophages (viruses) transferring bacterial DNA, not plasmids like R-factors. It does not involve direct cell contact or pilus formation.
Option B: Transformation is the uptake of free DNA from the environment, not direct transfer via conjugation. R-factors are not typically acquired this way.
Option D: Vertical transmission refers to inheritance from parent to offspring during cell division, not intercellular gene transfer. R-factors are not passed vertically in bacteria.

**Clinical Pearl / High-Yield Fact**
Conjugation is the only method of horizontal gene transfer that allows stable, plasmid-based resistance spread in bacterial populations—making it a key driver of antibiotic resistance in clinical settings.

✓ Correct Answer: C. Conjugation