**Core Concept**
Horizontal gene transfer is a mechanism by which bacteria can share and acquire resistance genes, allowing them to spread their ability to resist multiple drugs. This process is distinct from vertical gene transfer, where resistance genes are passed down from parent to offspring.

**Why the Correct Answer is Right**
The correct answer is conjugation, a type of horizontal gene transfer where bacteria directly transfer genetic material, including resistance genes, from one bacterium to another through a specialized structure called a conjugation pilus. This process allows for the rapid spread of resistance genes among bacterial populations, making it a significant concern in the context of antibiotic resistance. Conjugation is often mediated by plasmids, which are small, self-replicating circular DNA molecules that can carry multiple resistance genes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Transformation is another type of horizontal gene transfer, but it involves the uptake of free DNA from the environment, not direct transfer between bacteria. While transformation can also contribute to the spread of resistance genes, it is not the primary mechanism described in this question.
* **Option B:** Mutation is a vertical gene transfer mechanism where resistance genes are altered or created within a bacterium's own genome. While mutations can confer resistance, they do not involve the sharing of genes between bacteria.
* **Option D:** Transduction is a type of horizontal gene transfer that involves the transfer of DNA from one bacterium to another through a viral vector. While transduction can also spread resistance genes, it is not the primary mechanism described in this question.

**Clinical Pearl / High-Yield Fact**
Conjugation is a key mechanism by which antibiotic resistance genes are spread among bacterial populations, making it essential for healthcare providers to understand and address this issue in the context of infection control and antibiotic stewardship.

**Correct Answer:** C. Conjugation.