## Core Concept
Horizontal gene transfer (HGT) among bacteria refers to the transfer of genetic material between bacterial cells other than by vertical inheritance (parent to offspring). This process is crucial for bacterial evolution, adaptation, and acquisition of new traits, including antibiotic resistance. Various methods facilitate HGT, including natural transformation, transduction, and conjugation.

## Why the Correct Answer is Right
The correct answer, **C. Transformation**, is a natural method of horizontal gene transfer. Transformation involves the direct uptake and incorporation of free DNA molecules from the environment into a bacterial cell. This process is a natural ability of some bacteria, notably *Streptococcus pneumoniae*, *Neisseria gonorrhoeae*, and *Haemophilus influenzae*. The DNA can come from lysed bacterial cells or be deliberately released by competent bacteria. Once inside, the acquired DNA can integrate into the bacterial genome, potentially altering the bacterium's genetic makeup.

## Why Each Wrong Option is Incorrect
- **Option A: Transduction** is a method of HGT but involves the transfer of DNA from one bacterium to another through a viral vector (bacteriophage). While it's a significant mechanism, it's not considered a "natural" method in the context of direct bacterial-to-bacterial transfer without a vector.
- **Option B: Conjugation** is a process where DNA is transferred directly from one bacterial cell to another, usually via a specialized structure called a conjugation pilus. While it is a natural method of HGT, the question seems to focus on a method not requiring direct cell-to-cell contact or a vector.
- **Option D: Mutation** is not a method of HGT at all. It's a change in the DNA sequence within a gene or chromosome of an individual bacterium, which can result in the creation of a new character or trait not found in the parental type.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that transformation is notably significant in *Streptococcus pneumoniae*, where it plays a critical role in acquiring antibiotic resistance and virulence factors. Competent bacteria can take up free DNA from their surroundings, making transformation a key mechanism for genetic exchange in these pathogens.

## Correct Answer Line
**Correct Answer: C. Transformation**