D. All of the choices.
Plasmids, cosmids, and bacteriophages can all be used as vectors for cloning DNA fragments. These are commonly employed tools in molecular biology for the replication and propagation of foreign DNA sequences in host cells.
Plasmids are small, circular DNA molecules that exist separate from the chromosomal DNA in bacterial cells. They can be easily manipulated and introduced into bacterial hosts, where they can replicate independently.
Cosmids are hybrid vectors that combine the properties of plasmids and bacteriophages. They are derived from plasmids but contain DNA sequences from bacteriophages that allow for efficient packaging of larger DNA fragments.
Bacteriophages, also known as phages, are viruses that infect bacteria. Bacteriophage vectors are specifically designed to infect bacterial hosts and can be used for the cloning and propagation of DNA fragments.
All three types of vectors—plasmids, cosmids, and bacteriophages—offer distinct advantages and are utilized in various cloning applications depending on the size of the DNA fragment, the host organism, and other experimental requirements.
Therefore, the correct answer is D. All of the choices.