Disarmed pathogen of plants : Agrobacterium tumefaciens :: Disarmed pathogen of animals : __________.

The correct option is C Retroviruses
Pathogens are those organisms that cause diseases. Disarmed pathogens are created by removing the genes that are responsible for virulence and adding genes of our interest instead so that when the disarmed pathogen is used to infect the host, it does not cause disease but transfers the recombinant DNA into the host.

Agrobacterium tumefaciens is a bacterium which is a pathogen of several dicot plants. It contains a plasmid called Tumor inducing (Ti) plasmid. This bacteria naturally delivers a piece of Ti plasmid DNA called ‘T-DNA’ to the host. This leads to the transformation of normal plant cells into tumor cells to produce chemicals required by the bacterial pathogen. This Ti plasmid has now been modified into a cloning vector which is no more pathogenic to the plants but is still able to deliver genes of our interest into a variety of plants.

Retroviruses have the ability to transform normal animal cells into cancerous cells. Retroviruses have also been disarmed and are now used to deliver desirable genes into animal cells.

Bacteriophages are viruses that infect bacteria. They are extremely host specific and can not be used as disarmed pathogens in animals.

Plasmids are double stranded, circular, extrachromosomal DNA molecules found in bacteria.