Transduction, a process of genetic recombination in bacteria in which genes from a host cell (a bacterium) are incorporated into the genome of a bacterial virus (bacteriophage) and then carried to another host cell when the bacteriophage initiates another cycle of infection.

**Transduction in Bacteria**

Transduction is a process of genetic transfer in bacteria, where the transfer of genetic material occurs through the action of bacteriophages (or simply phages). Bacteriophages are viruses that infect bacteria and use them as host cells to reproduce.

**Bacteriophage-mediated Transduction**

Bacteriophage-mediated transduction occurs in two main types: generalized transduction and specialized transduction. In both cases, the bacteriophage plays a crucial role in transferring genetic material from one bacterium to another.

1. **Generalized Transduction:**
- Generalized transduction occurs when a bacteriophage infects a bacterial cell and mistakenly packages the host DNA instead of its own viral DNA during the assembly of new phage particles.
- This happens because during the lytic cycle of the bacteriophage, the bacterial DNA can be fragmented and packaged into the phage capsid.
- When this bacteriophage infects another bacterial cell, it injects the packaged DNA into the recipient cell.
- The donated DNA can then be incorporated into the recipient cell's genome through recombination.
- This process allows the transfer of any bacterial genes, including antibiotic resistance genes, virulence factors, etc.
- Hence, generalized transduction is an important mechanism for horizontal gene transfer in bacteria.
- Bacteriophages such as T4 and P1 are known to carry out generalized transduction.

2. **Specialized Transduction:**
- Specialized transduction occurs when specific regions of the bacterial genome are excised along with the phage DNA during the lysogenic cycle of a temperate bacteriophage.
- When the phage enters the lytic cycle, it mistakenly packages the excised bacterial DNA along with its own DNA into the new phage particles.
- This packaged DNA can then be transferred to another bacterial cell upon infection.
- The transferred DNA can integrate into the recipient cell's genome through recombination.
- Unlike generalized transduction, specialized transduction only transfers specific bacterial genes located near the integration site of the phage.
- Examples of bacteriophages involved in specialized transduction include lambda phage and P22 phage.

**Conclusion**

In summary, the process of transduction in bacteria is primarily carried out by bacteriophages. Bacteriophages can mediate the transfer of genetic material between bacteria through generalized transduction and specialized transduction. Generalized transduction allows the transfer of any bacterial genes, while specialized transduction specifically transfers genes located near the phage integration site. Therefore, option A (Bacteriophage) is the correct answer.